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EIA422 and EIA485 Data Signals
RS422
The electrical interface described in EIA422 (RS422) is a data
transmission standard for balanced digital signals. It allows for a
single transmitter device communicating to as many as 32 receiving
devices. This type of data signal is well suited to systems that
require that data be distributed to several points without a return
data path. Several companies offer camera telemetry controllers using
this data interface. Because there is only one transmitting device on
the network, this one may remain active at all times. There is no need
for the driver to go into a high impedance state to allow others to
"talk." This configuration using multiple drivers on the same wire pair
is exclusive to RS485 described below.
EIA485
RS485 differs from RS422 in the requirement of the transmitter devices to go into a high impedance (Hi-Z) state. This allows multiple transmitter devices to reside on the same wire pair. The software must dictate a protocol that allows only one device to transmit at any one time in order to prevent data crashes. Data wiring can use two wires or four wires. Using two wires, the system works in half duplex. This means that data is exchanged between to points sequentially. When a four wire system is used, the system may be full duplex. In many cases the system head end controller will continuously poll data to all remote devices on a transmit pair. The remote devices all respond (one at a time!) back to the head end. This property of the network rests solely in the hands of the software (firmware).
NOTES ON HiZ
The driver chips used in RS485 communications are capable of changing into their high impedance state very rapidly. On even short lengths of wire there can exist a residual voltage charge after a driver circuit turns off. This can interfere with circuits that are used to detect the Hi-Z state. It is very important that the copper communications lines be terminated with a resistor across the data wire pair. The best place to locate such as resistor is at the furthest electrical input device on the wire pair. For instance if several RS485 inputs are connected to one driver at a head end, the wire connection would loop across all inputs in a chain. The first and last devices in the chain would need to be terminated. Typically any value of resistor from 120 to 220 Ohms 1/4 Watt is sufficient to stabilize the signal line.
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