Rev 1. 1 05/08/19
- 7 -
TELSEC
©
MP2 User Guide
8.
At the MP2, connect the cable shield to the protective (protected) earth ground. The cable
shields must not be connected to earth ground anywhere else in the cabling network except at
this point.
3.6
Line Termination
1.
At each end of the Modbus cable network daisy-chain, there must be an end of line termination
resistor. Connect a 1/2W 120-ohm resistor across the D0 and D1 conductors of the end devices.
See Figure 7 below.
2.
If the MP2 is at one end of the Modbus cable network, then use the built in End of Line resistor.
Remove the cover of the MP2 and locate DIP switch in the front right corner of the board. SW3
is for COM1 and SW4 is for COM2. Press switch number 2 down (middle switch) to enable the
end of line terminating resistor. If the MP2 is not the last device on the Modbus cable network,
then move switch 2 to the up position. See Figure 6 and Figure 7 below. No more than two line
terminations are allowed per Modbus cable network.
3.7
Line Polarization
When there is no data activity on an RS-485 balanced pair, the lines are not driven and, thus susceptible
to external noise or interference. To insure the receiver line stays in a constant state, when no data
signal is present, some devices need to bias the network. Polarization of the pair must be implemented
at one location for the entire Serial Bus
. Generally, this point is at the master device such as the MP2.
Other devices must not implement any polarization. Refer to the manual of the device you are
monitoring to determine if line biasing is required. Remove the cover of the MP2 and locate DIP switch
in the front right corner of the board. Switches 1 &3 on dipswitch SW3 is for COM1 and Switches 1 & 3
on dipswitch SW4 are for COM2. Line polarization is enabled when both switches are in the down
position and disabled when in the up position. Switches 1&3 must be in the same position on the
dipswitch. Refer to Figure 6.
Figure 6 - Modbus End of Line and Line Polarization Jumpers
COM 1
COM 2
