American Fibertek MRX-480 - Manual (Page 6)

6
DATA BIAS / TERMINATION SWITCHES
NOTE: This unit is shipped with the data bias and termination switches in the off position .
Switches are available externally that allow offset bias and termination features to be activated when
using RS485 data. These bias / termination switches, labeled S2, are located on the left side of the unit
and can be modified without opening the unit. These switches also allow termination features to be
activated when using RS422 data. When transmitting RS232 or Manchester data, the bias and
termination switches must remain in the off (up) position. Please refer to the drawing below for changes
to the default switch settings of switch bank S2.
Please remember when using offset bias switches that they must be used in pairs. If switch # 1 is down
then switch # 3 must also be down. 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 (down)
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 DATA
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.