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.