American Fibertek RR-94882SL - Manual (Page 9)

9
DATA TERMINATION AND BIAS SWITCHES
NOTE: This unit is shipped with data terminations off and data bias off for Data Channel
1 and Data Channel 2. For other configurations, please refer to the DATA TERMINATION
AND BIAS section on the next page for changes to the default switch settings.
Switches are available internally on the RT-482-1470/1490 and RR-82-1470/1490 units that
allow offset bias and termination features to be activated when using RS485 data. These
switches also allow termination features to be activated when using RS422 data. In order to
reconfigure the RS485/422 channel, the rack card cover needs to be removed. To access the
switches, remove the four screws on the top of the cover. The configuration switch banks are
located next to their corresponding data terminal block connector.
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.