RS-422 serial transmission

- an overview of the RS422 standard that is widely used for the transmission of high speed serial data

Transmissions using RS-232 are limited in their speed and the length over which data can be transferred. Normally the maximum is 19.2 k baud and the distance 15 metres, although for slow transmission speeds longer lengths can sometimes be used. However care must be taken because stray pick up becomes a problem as the length is increased. This can result in the data becoming corrupted even when slow transmission speeds are used.

To enable high speed data to be transmitted another specification was devised. Known as RS-422 ( RS422 ), it allows for speeds of 10 M bits per second to be achieved. However the distance over which data can be transmitted at this rate is limited to 50 feet. The overall maximum distance over which data can be transmitted is 4000 feet and for this length the data rate is limited to 100 kbps

To achieve much greater data speed, balanced transmission techniques are used within RS-422 and as a result differential drivers and receivers are required. Lower voltage line levels are used for RS422 than those used for the older RS232 standard. A space is represented by a line voltage level in the band between +2 and +6 volts while a mark is represented by a voltage in the range -2 to -6 volts. The range between +2 and -2 volts provides a good noise margin for the system. Additionally the RS422 standard allows for line impedances down to 50 ohms while supporting the high data rates.

To enable the differential driver to be used, the RS-422 standard uses a four conductor cable. Additionally up to ten receivers can be placed on a single cable, providing a multi-point network or bus.

Although RS422 is significantly different to RS232, it can often be used as a direct interface in many instances.


RS422 specification overview


Attribute Specification
Cabling Single ended
Multi-drop
Number of devices 5 transmitters
10 receivers
Communications modes half duplex
full duplex
Maximum distance 4000 feet @ 100 kbps
Maximum data rate 10 Mbps @ 50 feet
Signalling Balanced
Mark (data = 1)
condition
2V to 6V (B greater than A)
Space (data = 0)
condition
2V to 6 V (A greater than B)
Driver output current capability 150 mA

Although RS232 has been the most popular standard for serial data transmission, the higher speeds offered by RS422 are a distinct advantage and as a result, it is being used more widely. In addition to this, it can be used almost interchangeably with RS232 and this provides a significant advantage.

By Ian Poole


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