American Fibertek MRM-1485-2F8 - Manual (Page 7)

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-1485-2F8 using switch # 1. A termination resistor for RS485 output
data can be selected in the MRM-1485-2F8 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-1485-2F8 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.