Serial data transmission standards including RS232, RS422, RS423, and RS485 were widely used for many data links, proving effective connectivity for the day. Although not nearly as widely used today, they can still be found in some areas.
Serial data transmission links used cables with a variety of different functions to enable data to be sent reliably between two equipments. Although a number of different wires were often used within a cable, they did not require nearly as many as the systems that sent parallel data.
The first of the RS standards was RS232, or more correctly RS-232. This was developed in 1962 when the need for forms of transmitting data from modems attached telephone lines to remote communications equipments became apparent.
The ‘RS’ stands for Recommended Standard, although later these standards were formally adopted by the EIA / TIA in the USA.
The EIA is the Electrical Industries Association and the TIA is the Telecommunications Industries Association. Once RS-232 was established an equivalent standard was written for the ITU (International Telecommunications Union) to provide a more international standard. This would enable the same standards to be used worldwide and also give manufacturers access to a global market using just one product. This standard was known as V.24 and is totally compatible with RS-232.
With RS232 well established and the need for faster communications and over longer distances, further standards beyond RS232 were introduced. Although a number of standards were introduced, the most widely used are RS-422 and RS485.
Serial data standards summary
RS232 RS-232 is the most widely used serial standard that is in use. Many laptop computers incorporate a serial interface, and it was also used on many printers, although much less so now.
RS422 This standard gives a much higher data rate than RS232, but it uses differential transmission techniques. Many RS422 devices are compatible with RS232.
RS485 RS-485 is a standard that allows high speed data transmission along with multiple transmitters and receivers and this makes it able to be incorporated as a network solution
|Number of devices||One transmit and one receive||Five transmitters and ten receivers||32 transmitters and 32 receivers|
|Communication mode||Full duplex||Full duplex / half duplex||Half duplex|
|Maximum distance||50 feet at 19.2 kbps||4000 feet at 100 kbps||4000 feet at 100 kbps|
|Maximum data rate||19.2 kbps at 50 feet||10 Mbps at 50 feet||10 Mbps at 50 feet|
|Mark (1)||-5 to -15 V||2 V to 6 V max. (B>A)||1.5 V to 5 V max. (B>A)|
|Space (0)||+5 to +15 V||2 V to 6 V max. (A>B)||1.5 V to 5 V max. (A>B|
|Output current capability||500 mA||150 mA||250 mA|
Each of these standards meets a different requirement. RS232 still being very widely used despite the fact that it has been in use for over 40 years. However the other standards that have been introduced more recently provide higher levels of performance that are very useful in may applications.
RS232 serial data transmission
The data is sent serially on RS232, each bit is sent one after the next because there is only one data line in each direction. This mode of data transmission also requires that the receiver knows when the actual data bits are arriving so that it can synchronise itself to the incoming data. To achieve this a logic 0 is sent as a start bit for the synchronisation. This is followed by the data itself and there are normally seven or eight bits. The receiver obviously has to know how many data bits to expect, and there are often small dual in line switches either on the back of the equipment or inside it to set this information.
Data on RS232 is normally sent using ASCII (American Standard Code for Information Interchange). However other codes including the Murray Code or EBCDIC (Extended Binary Coded Decimal Interchange Code) can be used equally well.
After the data itself a parity bit is sent. Again this requires setting because it is optional and it can be even or odd parity. This is used to check the correctness of the received data and it can indicate whether the data has an odd or even number of logic ones. Unlike many systems these days there is no facility for error correction.
Finally a stop bit is sent. This is normally one bit long and is used to signify the end of a particular byte. Sometimes two stop bits are required and again this is an option that can often be set on the equipment.
RS232 data transmission is normally asynchronous. However transmit and receive speeds must obviously be the same. A certain degree of tolerance is allowed. Once the start bit is sent the receiver will sample the centre of each bit to see the level. Within each data word the synchronisation must not differ by more than half a bit length otherwise the incorrect data will be seen. Fortunately this is very easy to achieve with today’s accurate bit or baud rate generators.