American Fibertek RR-91685SL - Manual (Page 9)

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The RT-482-1470/1490 and RR-82-1470/1490 units are shipped with these switches in the off
position. When transmitting RS232 or Manchester data, the bias and termination switches must
remain in the off position. The chart below describes the layout of the S1 switch bank. The
switch bank for data channel 2 follows the same pattern.
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.
RS485 OFFSET BIAS
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.