7
RS485 TERMINATIONS
The RS485 protocol is an expanded version of the original RS422 protocol. The RS485
protocol is an expanded version of the original RS422 protocol. RS485 differs from
RS422 in the ability 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 one device to transmit at any one time 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 two 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 from all remote devices. The
remote devices all respond back to the head end (one at a time!) as they are addressed.
This property of the network rests solely in the hands of the software or firmware.
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 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 resistors across the data wire pair. The best place to locate such
resistors is at the furthest electrical devices at the ends of the wire pair. For instance, if
several RS485 devices are connected in a daisy chain fashion, the wire connection
would loop across all devices in a chain. The furthest two points in the chain would need
to be terminated. A termination resistor for RS485 input data can be selected in the
MRM-1485 using switch # 1. A termination resistor for RS485 output data can be
selected in the MRM-1485 using switch # 2.
OFFSET BIAS – RS485
The RS485 specification requires receivers to detect input signals down to 200mVp-p of
voltage level. In many cases this can cause systems to be sensitive to noise on the data
wires. In an effort to eliminate the effects of low levels of noise, some manufacturers of
equipment that communicate using RS485 have introduced a small voltage bias to the
data lines. This is usually accomplished using 470 Ohm resistance to +5V on the
positive line and 470 Ohm resistance to ground on the negative line. When used in
conjunction with the appropriate termination resistors referred to in the previous section,
this introduces about a 300 mV offset, improving noise immunity.
Offset bias for RS485 can be applied on the MRM-1485 by setting switch # 3 and switch
# 4 on. Please note that these switches work as a set and must be either both on or both
off. In a majority of cases, bias will not be required and these switches should remain in
the off position. If using offset bias, it is important that termination resistor switch # 2 be
on (applied) position. Using offset bias without a termination on the line will cause
communications to fail.
