Varec 2920, Инструкция по установке и эксплуатации, Страница: 119 / 194

7 MODBUS
Varec, Inc. 109
Warning! Obtain a hot permit before removing the transmitter cover with power
applied.
To terminate the network line at the transmitter:
1. Remove the transmitter cover.
2. On the communications circuit board, set switch SW9 to the TERM RES (down) position.
3. Replace the transmitter cover.
Note Typically, no more than one transmitter on a communications bus should termi-
nate the network.
Setting the Bias Current
In a MODBUS network, during periods of no communications, all devices on the network are
in high-impedance states. This causes the data+ and data- lines to “float” meaning that they
may not have a fixed voltage.
To remedy this situation, pull-up and pull-down resistors are typically used to provide a small
bias current on the network. This current holds the data+ and data- lines at predetermined
high and low voltage settings. Bias current is normally provided at the host, but can be
provided at the 2920 FTT using switch SW8.
Warning! Obtain a hot permit before removing the transmitter cover with power
applied.
To apply the bias resistors to the MODBUS network:
1. Remove the transmitter cover.
2. On the communications circuit board, set switch SW8 to the BIASED (down) position.
3. Replace the transmitter cover.
Note Applying the bias current on multiple 2920 FTT transmitters could cause poor per-
formance in the EIA485 communications network.