Interfacing is the process of connecting a microprocessor to the rest of the circuit or to external devices. Even in the simplest of computer systems, there is some input device like a keyboard. So how does the microprocessor know that we have pressed a key? When we send text to a printer, how does the printer tell us that it is ready for more input?
In a general purpose microprocessor-based system, if it is to do anything useful, there must be inputs and outputs. The external devices must therefore communicate with the microprocessor. In some cases, the microprocessor takes the matter into its own hands and sends data out as part of its program but even in this case it normally allows the external device to help.
If a microprocessor-based system were used to heat some water, it is easy enough to imagine the program switching on the power supply and sitting there doing nothing for 10 minutes. It would be a better use of the microprocessor to leave the heater running and wait for a thermostat to signal that the water has reached the required temperature. This thermostat signal would arrive at an interrupt pin on the microprocessor.
Interrupts were introduced in Chapter 8 when we looked at the operation of the interrupt flag in the status or flag register but we will now delve a little further into the system.
All microprocessors have interrupts that can be initiated either by the software being run at the time or by external hardware circuits. Microprocessors differ in the details of their response to hardware interrupts and in the number of different interrupt pins offered. Details are always itemized in the technical data supplied with the device.
The likely options are as follows
There are two basic types of hardware interrupt. The first is an interrupt request or IRQ (or INTR) pin. This tells the microprocessor that it would like to have some attention. Since it is a request rather than an order, the microprocessor is free to say ‘yes’ or ‘no’ or ‘yes, but not at the moment – just wait till I’m ready’. This does not imply any intelligence on the part of the microprocessor – it must be told which response to give by the software that is being run at the time. In the absence of any instruction, it normally accepts the interruption. If a particular interrupt pin has been told not to respond to an interrupt, we say that the interrupt has been ‘masked’.
The second type is called a non-maskable interrupt, that is, an unstoppable demand for attention. This will always assume top-priority. A typical use for this would be for an emergency shutdown in the event of a power failure. The program can also instigate an interrupt by means of a software instruction as part of a program.
What is a hardware interrupt signal?
This is a change of voltage on an interrupt pin generated by the external device. The change required would be detailed in the technical data but there are four choices.
The first two choices are changes of voltage level. The pin can sit at +3.3 V or whatever the ‘high’ voltage happens to be, then responds when it falls to 0 V. We call this ‘active low’. When the pin goes low, an interrupt is recognized. Alternatively, it could sit at 0 V and become activated by an increased voltage. This we call ‘active high’.
The alternative approach is to use the sudden change of level. From low to high is called ‘leading edge’ or ‘rising edge’ and from high to low is the ‘trailing edge’ or ‘falling edge’ (See Figure 17.1 and have a glance at Figure 6.4 to see the alternative names.)
Figure 17.1 Four ways of signalling an interrupt
Once an interrupt is activated, the signal must be returned to its normal voltage before it can be triggered again. The input is masked as the interrupt program is running to prevent the interrupt pin from interrupting itself if the voltage remains at its active level.
Accommodating several external devices
The simplest and quickest way of connecting devices to the interrupts of a microprocessor is to have each device connected to its own interrupt pin. This is OK providing there are enough pins. Few microprocessors have more than two interrupt pins so we have to connect several devices to the same pin.
When an interrupt occurs, a program called the ‘first level interrupt handler’ or FLIH is activated. The function of the FLIH is to identify the device causing the interrupt and to pass the controls over to the ‘interrupt handler’ or ‘interrupt service routine’ program that has been written to deal with that device.
How does it know which device is crying for help? There are two options, polling and vectored interrupts.
Polling interrupts
This is a slow but sure way. Each possible device is interrogated in turn with an ‘is it you?’ signal until the source of the interrupt is found. The order of checking is prioritized so the most important device is checked first.
Vectored interrupts
Immediately after the interruption has occurred, the FLIH puts out a ‘who’s there?’ signal and the interrupting circuit puts an identification signal onto the data bus. This signal is usually used as part of an address to identify the section of program to be executed.
What happens if an interrupt is received while the previous interrupt is being dealt with? Again we have a choice. We can disable the new interrupt until the first one is complete, then deal with the new one. Alternatively, we can check the priority of the new alarm and decide on the new priorities. A higher priority causes the present interrupt to be halted and its current state to be saved in the stack while the new one is worked on and then we return to unload the stack information and carry on with the original problem. If the new interrupt is less important it gets a ‘wait a bit’ message until the first one is finished.
In a microprocessor-based system it is up to the designer to decide on the priorities and uses of the interrupts. In a PC, the interrupts are prioritized in order of speed and importance. Top priority is given to the internal clock. This is the clock that tells us the time – not the square wave clock that synchronizes the circuitry. Thereafter, in order, we have the keyboard, two spare ones for any other operations, then comes the serial port, the hard drive, the floppy disk drive and finally the printer.
If you were to walk into a crowded room and say ‘Burgers’ and nothing further, many of those present would turn to their neighbours and say ‘What was that?’ and some would just stand and stare. (Some may even mis-hear and feel offended.) As a form of communication, this is not very efficient. Try instead, walking in and saying ‘Lunch will consist of burgers’. Everyone would understand your message.
The first attempt was very efficient in terms of the number of words used but is probably likely to be inefficient communication since many people will not receive the message. In the second attempt, we have used five words to make sure that the one important one gets through. This is called adding ‘redundancy’. The more redundancy we add, the more certain is the message but the slower and less efficient becomes the communication system. Data being returned from space probes use very high levels of redundancy, over 96%, which allows for correction of really scrambled signals due to the extremely low power levels involved.
We can use parity for alerting us to the possibility of an error in a stream of data or, in some cases, we can detect and correct the error. In its simplest form, we take a group of bits in a transmission, 4 or 8 bits are normally used though the idea is applicable to other values. In this example, we will look at a 4-bit group, say 1001. At the transmitting end, we add an extra bit on the end, either a 0 or a 1 to make the total number of ‘1’s an even number. In this case, there are two ‘1’s and so the number is already even, so we add a zero. The data now reads 10010. At the receiving end, if the data has been mutilated and now reads 11010, a quick count will show that there is an odd number of ‘1’s and so an error has occurred.
This simple approach can be easily fooled. If there are two errors there will be an even number of ‘1’s and passed as correct. And, another disappointment, if it shows an error, we cannot tell which bit is wrong and therefore cannot correct it. When this system is used, an error signal is sent back to the transmitter requesting a repetition but this assumes that the transmitter and the receiver are in communication with each other. We can modify the system to make limited automatic correction feasible.
Let’s assume we have, say, 16 bits of data to send.
Step 1 Rewrite the data in the form of a square:
0 0 1 0
1 1 1 1
0 1 0 1
1 0 1 1
Step 2 Add parity bits. Across the top row, we have the numbers 0010 which includes a single ‘1’. In this system, which we will call ‘even’ parity, we add another ‘1’ if necessary to ensure that there is an even number of ‘1’s across the first row. As we have only a single ‘1’, we add another ‘1’ on the end. It now looks like this:
0 0 1 0 1
1 1 1 1
0 1 0 1
1 0 1 1
The top row now has an even number of ‘1’s. The next row has four ‘1’s which is an even number so we do not need to add another ‘1’. We therefore add a ‘0’:
0 0 1 0 1
1 1 1 1 0
0 1 0 1
1 0 1 1
The third row will be completed with a ‘0’ since it contains an even number of ‘1’s and the last row, with three ‘1’s will need an extra one to be added. The result is now:
0 0 1 0 1
1 1 1 1 0
0 1 0 1 0
1 0 1 1 1
Step 3 We now have five columns down the page and we can add extra ‘1’s in the same way to make the total number of ‘1’s an even number. The first two and the last columns each contain two ‘1’s so zeros will be added. The third and fourth columns have three ‘1’s so we need to add an extra ‘1’ to each. The result is now:
0 0 1 0 1
1 1 1 1 0
0 1 0 1 0
1 0 1 1 1
0 0 1 1 0
Notice how we have now got a total of 25 bits to be transmitted. This represents 16 bits of data and 9 bits added to check the accuracy of the data. The final serial transmission is 0010111110010101011100110. This means that 9 out of 25 or 36% of the transmission is not actual data and represents redundancy.
Let’s see how it works. We will assume an error has occurred and one of the bits is received incorrectly so here is the received transmission:
0010111110010101001100110
Step 1 Layout the data as a 5×5 square.
0 0 1 0 1
1 1 1 1 0
0 1 0 1 0
1 0 0 1 1
0 0 1 1 0
Step 2 Check the parity in each row across the square. We decided to make each row and column to have an even parity.
The first row has two ‘1’s, this is even – OK.
The second row has four ‘1’s, this is even – OK.
The third row has two ‘1’s, this is even – OK.
The fourth row has three ‘1’s, this is odd – an error has occurred.
The last row has two ‘1’s, this is even – OK.
We now know that one of the bits in the fourth row has been received incorrectly.
Step 3 Do the same for the columns.
The first column has two ‘1’s, this is even – OK.
The second column has two ‘1’s, this is even – OK.
The third column has three ‘1’s, this is odd – an error exists in column three.
The fourth column has four ‘1’s, this is even – OK.
The last column has two ‘1’s, this is even – OK.
Step 4 Isolate the error and change the data.
The error occurs in the third column and the fourth row. Since this is now known to be an error and we only have a choice of 0 or 1, we can confidently change the 0 to a 1 and recover the correct data stream.
0 0 1 0 1
1 1 1 1 0
0 1 0 1 0
1 0 1 1 1
0 0 1 1 0
In this example, we chose to use even parity, that is, we made each row and column have an even number of ‘1’s. It would work equally well if we used odd parity by making the number of ‘1’s an odd number. It would also work just as well if we counted the zeros instead of the ones. If more than one error occurs, it will warn us of an error but it will be unable to make any corrections. If you try it, you will see that it indicates four possible positions for the two errors and nine for three errors.
Example
Correct this received data which includes one error. To provide automatic correction, odd parity on the ‘1’s has been used.
The received signal: 1101001001111001010101101.
Step 1 Layout the data as a 5×5 square
1 1 0 1 0
0 1 0 0 1
1 1 1 0 0
1 0 1 0 1
0 1 1 0 1
Step 2 Check the columns and rows for an odd number of ‘1’s
1 1 0 1 0 ✓
0 1 0 0 1 ×
1 1 1 0 0 ✓
1 0 1 0 1 ✓
0 1 1 0 1 ✓
✓ × ✓ ✓ ✓
Step 3 Isolate the error
1 1 0 1 0 ✓
0 1 0 0 1 ×
1 1 1 0 0 ✓
1 0 1 0 1 ✓
0 1 1 0 1 ✓
✓ × ✓ ✓ ✓
Step 4 The data where the column and row intersect is the error. So we simply change the ‘1’ to a ‘0’.
Step 5 Strip out the parity bits to recover the original data
11010000111010100110
The most basic way of sending information from one place to another is simply to connect a wire to both ends of the system and apply a voltage to one end. By making the voltage vary, we can send different levels and even speech or music. These are called analogue signals and have many drawbacks.
The main one is the effect of noise. As the signal travels along a wire it gets weaker and it has noise induced into it by random electromagnetic signals and vibration of the molecules of the conductor. The overall effect is that the signal becomes degraded and weaker.
The ‘weaker’ bit is no problem, we can soon amplify it back to its original size but the noise is a different matter. The electrical noise has become embedded into the signal and has permanently distorted it. Amplifying it will amplify the noise and the signal equally.
We have no problem with digital signals since we know that they will all be rectangular in shape and so the amplifier can be used to regenerate the shape of the signal and hence strip out the effects of the noise. If we are faced with sending something inherently analogue, like speech or music, our first job is to convert it to a digital form.
Inevitably these days, this is taken care of by an integrated circuit of which there are many different designs. It is quite possible, but totally uneconomic, to construct our own ADC so it is really a matter of flicking through the catalogues and choose the most appropriate one available.
There are several different designs of ADCs, which are based on three basically different approaches.
Flash converter
The first is called a flash converter or parallel encoder. These use circuits called comparators. A comparator has two inputs, one is the analogue voltage being converted and the other is a known reference voltage.
All we ask of a comparator is to answer a simple question: ‘Is the analogue input voltage higher or lower than our reference voltage?’ It answers by changing its output voltage to a logic 1 to mean it is higher and a logic 0 to mean it is lower. They are so accurate that the chance of it accepting the two voltages as the same level are extremely slight and doesn’t happen in practice (see Figure 17.2).
Figure 17.2 A comparator used in a flash ADC
So how do we use the comparator? If we had three of them with reference voltages set to 1 V, 2 V and 3 V and then applied the input voltage of 2.5 V to all of them, the first two would set their outputs to a level 1 and the last one would be unaffected at logic 0. The logic levels could be used to generate a binary number to represent 2.5 V.
An input voltage of 2.4 V or 2.9 V would also result in the same comparators being activated and hence the same output digital signal. This error occurs in all analog to digital converters. We can reduce the size of the error by increasing the number of comparators to detail more levels. Real ones have between 16 and 1024 different levels.
Ramp generators
These are a combination of a binary counter that simply counts up from zero to its maximum value, perhaps 1024 like the last type. As the binary count proceeds, a ramp voltage is made to steadily increase. A single comparator is used to compare the output of the ramp voltage with the analog voltage being converted. As soon as the ramp voltage exceeds the input voltage, the comparator signal stops the counter. The counter output is then the digital equivalent of the analog signal (see Figure 17.3).
Figure 17.3 An ADC that uses a ramp voltage
Successive approximation
If we were to use a 3-bit digital signal to convert an analog voltage of between 0 V and 4 V we could have the 3 bits representing voltages of 4 V, 2 V and 1 V. This is how the circuit responds to an input of 3.5 V.
The digits are initially set to 000. The left-hand bit is switched on and its 4 V is compared with the input. The input is seen to be less than this so this digit is reset to zero. It then tries the next bit and its 2 V are compared and found to be less than the input so it remains set. The digital signal is now 010. The circuit now adds the 1 V from the last digit. The result is a total of 3 V, which is compared with the input analog signal. The input of 3.5 V still exceeds the current value of 3 V so the last bit is set. The final digital output is 011.
The circuit has tried all the available values until it finds the one that provides the result closest, but less than the input signal. As before, the more bits we are using, the more accurate is the result.
In checking the specifications of likely ADCs to use, we need to compare the following criteria.
Quantization error
In the above example using the flash converter, we can see that an analog input of 3.5 V would provide the same output as would any value between slightly over 3 V and slightly less than 4 V. This error means that small variation in the analog input voltage will be lost. The size of this error is equal to the space between the comparator reference voltages.
Changing from eight comparators to 1024, would mean that the voltage gaps would decrease from 1 V to 7.8 mV as would the quantization error. Regardless of the method used for A–D conversion, quantization error is always present.
Bits
The more bits, the merrier. Likely values will be between 8 and 16.
Speed
There are two factors here. How often can we get an updated value for the signal and how well can we follow any changes it is making? Even if we do not want the signal to be sampled at a very high rate, we still may want to take a quick sample so that the input value is unlikely to change very much as the sample is actually being measured.
For speed, you cannot beat the flash converter. It can sample for a period as short as 3 ns which compares very favourably with the typical values of 10 μs for the ramp generators and successive approximation types.
Changing a group of digital bit values to an analog voltage is basically just the reverse process of the A–D conversion that we met in the previous section.
Most digital to analog converters operate by adding current together then converting the result into an analog voltage. The binary levels are used to switch currents on or off.
Let’s assume a 4-bit digital signal in which the most significant bit is made to generate a current of 8 mA and the others produce, in turn, 4, 2 and finally 1 mA. If the digital signal to be converted happened to be 10112, then the first, third and fourth current sources would be activated giving a total of 8+2+1=11 mA (Figure 17.4).
Figure 17.4 A DAC with a current or voltage output
In some DACs the final output is a changing current but in others it has been converted to a variable voltage. It just depends on which integrated circuit you choose to use. In the ones offering a voltage output, the total current is then passed through a resistor. If we choose a nice easy value like 1 kΩ, the voltage across the resistor would be 11 mA×1 kΩ=11 V.
In a similar way, we can see that all binary values between 0000 and 1111 would be converted into voltages between 0 V and 15 V. There are a couple of specifications that may need to be checked to decide on which one to use.
Resolution
This is the number of digital bits used to convert into an analog voltage. Typical values available are from 4 to 18 bits. As the digital input changes by a single bit, say from 1000 to 1001, the resultant voltage or current increases by a discrete step. The size of this step is determined by the number of bits used compared with the maximum value of the output current or voltage.
For example, if we used 4 bits then this would provide a total of 16 different steps and if the maximum happened to be 8 V, each step will represent a voltage change of 0.5 V. Thus, a steadily increasing digital signal will cause the analog voltage to increase in small discrete steps like a staircase. This is all very similar to the cause of quantization error.
Speed
The speed of operation is very dependent on the chip being considered. The conversion times available from an exceedingly fast 1 ns to a sluggish 5 μs.
In sending information in digital form, we have a choice of using serial or parallel transmission. In the case of serial transmission, the binary values are represented by two different voltage levels and are sent one after the other along a cable. This is simple but slow. The alternative is to have several wires and thus be able to send several bits of data at the same time, one on each conductor.
In a microprocessor-based system, even if it is a one-off, it is usually better to conform to established standards for the cabling so that other instruments and circuits can be connected with a minimum of hassle.
Parallel connection
There are several different standards used for parallel connection of data but one of the most widely used, and most reliable, was produced by Centronics.
The Centronics system sends eight bits at a time and employs a 36 plug and socket system. To send data, there are four basic control signals as well as the eight data lines. It also stipulates a variety of other control wires that can be used if required.
The important thing to remember about these standards is that you do not have to use all the connections listed but those that you do decide to use should conform to the stated specification and be on the correct pins. This ensures that if the plug is inserted into a new piece of equipment, it may not work but at least it will not be damaged.
Centronics data transmission
To see how the system works, we will use timing diagrams to show what happens and when. These diagrams, which all look very similar at first glance, are shown in all data manuals to show the sequence of events inside the microprocessor and in the surrounding circuit.
There are a couple of points that are worth mentioning. We have mentioned the problem of rise time in Chapter 7. You will remember that we cannot change a voltage level instantaneously. It may not seem an important delay when we think of switching a light on at home but when the microprocessor is handling data at millions of bits per second, these delays can be important and is a common cause of failure in a circuit that ‘should’ work. Most waveform diagrams show the ends of a square wave as sloping lines rather than vertical ones.
In Figure 17.5 we have a positive-going pulse to represent a data value of 1 but, of course, data could equally well be at 0 V to represent a binary 0. In cases where we want to show that a level has changed, but it may go to either level, we redraw the diagram to show both possibilities at the same time, as in Figure 17.6.
Figure 17.5 Rise and fall time may be important
Figure 17.6 Showing alternative data levels
Figure 17.7 shows the process of transferring eight bits of data from a microprocessor to an external printer or other device.
Figure 17.7 The timing of Centronics signals
Step 1 The microprocessor puts the eight bits of data on the data wires.
Step 2 A short delay occurs while we wait for the data voltages to settle on all the eight wires. Then the strobe pulse occurs to tell the printer or other accessory that the data is ready. The line over the word strobe indicates that it is active low. No line would mean active high.
Step 3 The printer starts loading data and the busy line goes high to prevent more data being sent.
Step 4 When the data has been printed, the busy line goes down to tell the microprocessor to put the next piece of data onto the data wires.
The standard allows for many other control wires for other purposes. In this example, the eight data wires have been used as outputs from the microprocessor but it is quite possible to use them to carry input data although this is not allowed for in the design of PCs.
To send information in serial form requires only a simple communication link but it is inevitably slower than parallel transmission since the data is only sent one bit at a time.
UARTs
To convert the parallel data on the data bus of the microprocessor to a serial transmission we could use a shift register as in Chapter 6. The modern alternative is to use a chip called a UART (universal asynchronous receiver/transmitter) or USART (universal synchronous/asynchronous receiver/transmitter).
These are integrated circuits that convert data from parallel to serial transmission so instead of having eight wires, each carrying a single bit at the same time, the serial transmission passes the bits, one at a time along a single wire. It can also receive eight bits of serial data then pass it into the microprocessor in a single byte. Parallel transmission is obviously a lot faster since eight bits are moved at a time but it requires eight connections. To send a fax signal, for example, would require the use of eight telephone lines whereas the UART can convert it to a serial transmission and sent it over a single line at one-eighth of the speed.
UARTS do a lot more than a shift register. They include parity checking and buffers to enable it to handle about 16 bytes at a time without involving the microprocessor. Most transmissions involve the ASCII code to represent the characters to be transmitted. This is a seven-bit code to represent each alphanumeric character and a variety of control instructions. For example, the letter E is 45 in hex which, using only the lower seven bits, is 1000101. The ASCII code is used in both parallel and serial transmissions. Each letter and symbol has its own seven-digit code. A further bit is added on the end to provide a parity bit or it can be used to swap over to an alternative set of characters to allow mathematical symbols and Greek letters to be transmitted or, if unused, can be left at zero. Our letter E would then be represented as shown in Figure 17.8.
Figure 17.8 Coding in ASCII
When using ASCII signals in a serial transmission, we need to be able to tell the receiving apparatus when a particular ASCII character has been sent. This is easily done in a synchronous system that ensures that the transmitter and the receiver are locked together running at the same speed. This is not the easiest way of operating the system owing to the difficulties of ensuring the two devices remain synchronous. Therefore, we tend to operate asynchronously. This means that we have to send a signal along with each ASCII code to tell the receiver when the code has started and when it has stopped. Otherwise the transmitter would send a continuous stream of data and if a bit were lost, the receiver would get out of step and would misread all subsequent data.
To get round this problem, a 0 V ‘start’ bit is sent at the beginning of the character and a positive ‘stop’ bit is sent at the end. This brings a seven-bit ASCII code up to a total of 10 bits. The start and stop bits ensure that there is at least one change of level for each character that can be used to keep the receiver clock nearly synchronized to the transmitter for the time taken to receive that character.
For distances over a few metres, we need to use a slightly more sophisticated transmission system to prevent random noise from interfering too much. There are several systems in use, the most popular being those created by the EIA (Electrical Industries Association).
As with most transmission media, there is a trade off between the speed and the maximum distance the system can be used for. If you intend pushing the transmission distance to its maximum value, you will have to accept a reduced speed. As a rule of thumb, halve the speed if you double the distance.
RS232C
This is one of the transmission standards created by the EIA committee. This standard allows for transmissions up to 50 feet (15 m) and at speeds of up to 20 kbaud (it can actually exceed this speed and distance but it’s not guaranteed). The baud is the measure of the speed of transmission. It is the number of clock periods per second, which approximates to the number of bits per second. The RS232C transmission is balanced at about 0 V. Here’s the time to be careful, the binary one level is a negative voltage (between –5 and –15 V) and a binary zero level is a positive value between +5 and +15 V. This seems upside down compared with all our previous uses of binary. Our letter E would be transmitted as in Figure 17.9. The transmitter levels are specified as ±5 V but the receiver limits are ±3 V. This allows for a noise spike to be up to 6 V before there is any possibility of misreading a piece of data.
Figure 17.9 RS232C transmission
RS423A
This is an improved version having a maximum speed of 100 kbits/s and a maximum cable length of ¾ mile (1.2 km). The transmission voltages have to be between ±3.6 and 6 V and the receiver can go down to ±0.2 V.
Changing voltage levels
How do we change the binary or logic values into the RS232 voltage levels? If you are building a microprocessor-based system then the most obvious way is to use a pair of integrated circuits called the 1488 (transmitter) and the 1489 (receiver). These integrated circuits have been around for many years and are simple and reliable. They have a small snag in that they need 12 V supplies whereas nowadays 5 V supplies are much more common so you may find some new transceivers (made by Maxim) more interesting. These only require a single +5 V supply and generate their own ± voltages for the RS232C transmission. Each chip contains two transmitters and two receivers and operate up to 120 kbits/s. The devices are numbered MAX202, MAX208, MAX220 and MAX232 and others. PCs have a serial port that provides signals at RS232C levels.
Using RS232C in real life
Most RS232C links are via a 25-pin ‘D’ plug or a 9-pin ‘D’ plug and socket (Figure 17.10) but unlike the Centronics which is quite stable and usually work straight off, the RS232C can be a real nuisance. Before attempting to communicate, you must ensure that the transmitter and the receiver are using the same word length and parity values are set for the same speed of operation. Even then, it may take some experimenting before they spring into life. The problem is that there are many more options for the other connections. All have to be agreed between the receiver and the transmitter. The specifications are not detailed enough and can lead to different interpretations. It is not surprising that it is often insufficient to connect an RS232C cable between two pieces of equipment and switch on. You will need to get hold of the RS232C connection specification and settle down in a comfortable chair.
Figure 17.10 ‘D’ connectors for RS232C
Modems
A modem (MOdulator DEModulator) converts a digital signal into two audio tones so that the transmission can occur along a telephone line. Telephones are generally designed to accept frequencies between 300 Hz and 3.1 kHz. This relatively narrow bandwidth was chosen to allow speech to be transferred with undue loss of quality while allowing the largest number of calls to be passed along the same cable. Once the digital signals are on a telephone line then the range is unlimited.
A few metres
We can use the raw binary data transmitted over a simple cable (see Figure 17.11).
Figure 17.11 A very short range link up
Tens or hundreds of metres
We can convert the transmitted signal to RS232C or RS423A as necessary (see Figure 17.12).
Figure 17.12 Around the building
Unlimited range
Add a modem and link by telephone or optic fibre (see Figure 17.13).
Figure 17.13 Around the world
A piece of optic fibre is a solid piece of glass or plastic. The plastic fibre is about 1 mm in diameter and is suitable only for short ranges of a few tens of metres but it has the advantage of being cheap and easy to use. Its useful range is limited by the clarity of current plastics. The special silica glass is incredibly clear and hence has much lower losses and able to be used over any distance, with suitable repeaters. It also has a much smaller diameter – only about 125 μm before the external protective layers are added.
If a light is shone into the end of an optic fibre, it will reflect off the inner surfaces along the cable. The light source used is a laser operating in the infrared region of the spectrum. To use it as a means of sending a digital signal we need to switch the light source on and off and then detect the flashes of light at the far end of the cable by a photoelectric cell. The losses can be made up by repeaters just as we do on copper-based systems, so range of operation is no problem. The optic fibre does not suffer from any electric noise pickup along the route and has an enormous bandwidth. In one sense, it is not really optional because nearly all long distance telephone cables are now optic fibres. (See Further reading for our companion volume An Introduction to Fiber Optics.)
If we are constructing our own fibre optic link, all we need to do is to buy the laser (or light emitting diodes (LEDs)) and the photocells and some plugs and sockets to connect it all up. It can be used to replace the copper cable in any of the systems described (see Figure 17.14). Careful! The infrared light from the lasers can cause immediate and irreversible eye damage. We must always remember that we are down to our last pair of eyes.
Figure 17.14 A fibre optic link – any distance
Data transfer rates
Using a single optic fibre for serial transmission, typical data transfer rates of 100 Mbytes/s are available up to 10 km. Very high speed data transfer of 1 Gbyte/s can be achieved up to 100 m using parallel transmission along a bunch of fibre optic cables.
In each case, choose the best option.
1 The fastest design of analogue to digital conversion is a:
(a) ramp converter.
(b) flash converter.
(c) comparator.
(d) successive approximation converter.
2 Quantization error can be reduced by:
(a) increasing the number of levels in the ADC.
(b) increasing the speed of the conversion.
(c) using vectored interrupts rather than polling.
(d) using a flash converter.
3 Using the RS232C standard, a binary 0 is most likely to be transmitted as:
(a) –4 V.
(b) +2 V.
(c) –5 V.
(d) +10 V.
4 A modem is:
(a) a type of USART.
(b) normally connected between the UART and the RS232C converter.
(c) only used in fibre optic systems.
(d) used to convert digital signals into audio tones for transmission over telephone cables.
5 This received transmission has sixteen bits of data and includes an error. It is using odd parity on the ones in a block of 25 bits:
0000111010111000100110011.
The corrected data is:
(a) 1000111010111000101110011.
(b) 0000111010111000101110010.
(c) 0000111010111000101110011.
(d) 0000111010111000100110011.