high and vice-versa. Also, the drivers are capable of driving a wire with as much as 25 times more capacitance before the signal is badly distorted. Most system designers try to minimize the number of signals used because these standards take two wires per signal. This is one reason most RS-422 and RS-485 systems are either two-wire with ground or four-wire with ground.
There are two major differences between RS-422 and RS-485. The first is that with RS-422, one driver can only drive 10 receivers. With RS-485, one driver can drive 32 receivers. Secondly, RS-485 drivers can be “tri-stated” or turned off. This allows you to put all 32 drivers and receivers on one two-wire line. When a device needs to output data, it first turns on its driver to seize the line and then sends its data. The other 31 devices will be listening. RS-422 requires at least two pairs of wires, one pair to send data in one direction and the other for return data.
Typically, the RS-422 or RS-485 driver will switch between +5 volts and ground. Since devices can be long distances apart, they can be powered on different power lines or transformers that can force their “reference grounds” to be at different voltages. The RS- 422 Standard allows the signal lines to go as high as +7 volts and as low as -7 volts. The RS-485 Standard allows +12 volts and -7 volts.
Another requirement is that the differential input voltage at the receiver must be more than 0.2 volts. The differential voltage starts out from the driver at about 2 volts but the resistance, inductance, and capacitance of the wire combine to attenuate this signal. As the wire gets longer, this voltage will get smaller until it falls below the
0.2volt level and then communications can no longer be guaranteed.
Both specifications discuss termination of the two-wire line. A typical two-wire twisted-pair line looks like a 100 ohm transmission line. In general, the line does not need to be terminated for slow data rates or for short-wire lengths.
More information and the RS-232, RS-422, and RS-485 Standards are available from:
ELECTRONICS INDUSTRIES ASSOCIATION
Engineering Department
2001 Eye Street N. W.
Washington, DC 20006
Phone: (202) 457-4900 --- FAX: (202) 457-4985
2Documentation Number 485T1995 Manual
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5105
