American Fibertek MT-91P558 - Manual (Page 6)

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The MT-91P558 is shipped with these switches in the off (left) position. When transmitting RS232 or
Manchester data, the bias and termination switches must remain in the off (left) position. The top S1
switch bank corresponds to channel 1. The middle switch bank (shown below in exploded view)
corresponds to channel 2. To turn bias or termination functions on, slide the appropriate switch to the
right.
Please remember when using offset bias switches that they must be used in pairs. If switch # 1 is on
then switch # 3 must also be on. The same situation applies for switch # 4 and switch # 6. Also, if using
offset bias, it is important that the corresponding termination resistor switch be in the on position. Using
offset bias without a termination on the line will cause communications to fail. An explanation follows on
general bias and termination guidelines.
RS485 DATA TERMINATION
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
only one device to transmit at any one time to prevent data crashes. 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. The driver chips that are 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.
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 a 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.