MCM-Inst-1.10
Page 17 of 27
3.7.2 RS-485 Connection
RS-485 wiring can be complex when multiple devices are connected, when running wires for long
distances, and when using termination and biasing resistors.
●
The MCM RS-485 outputs are electrically isolated from dangerous voltages.
●
If the output wiring runs near line voltage wiring, use wires or cables with a 300 V or 600 V rating
(not less than the service voltage).
●
If the output wiring runs near bare conductors, it must be double insulated or jacketed.
●
RS-485 wiring may be daisy-chained between devices. Avoid a star or home run wiring system,
where each device is wired back to the Modbus master, as this can cause signal reflections.
●
You may install two wires into each screw terminal by twisting the wires together first.
●
Torque the screws to 3.5 lbf·in (0.4 N·m).
●
Note:
one loose wire can disable an entire network section.
3.7.3 Modbus RTU DIP Switches
The MCM-MB uses two DIP switches to configure the address, the baud rate, termination, and
biasing.
1 2 3 4 5 6 7 8
1 0
O N
1 2 3
1
0
O
N
Baud Rate
Address
Bias-
Bias+
Termination
Figure 10: DIP Switches
Use DIP switches 1-6 from the upper switch to select the base Modbus address (1 to 63). Each
submeter block has its own offset from the base address. The first submeter’s address is the base
address, the next is the base plus one (designated as “ADDR+1”), and so on.
Note: do not set DIP
switches 1-6 all off, since this would result in a base address of 0, which is invalid.
Table 2: Modbus Address DIP Switch
DIP Switch
1
2
3
4
5
6
On
(1) Value
1
2
4
8
16
32
Address
Examples
1
1, On
0, Off
0, Off
0, Off
0, Off
0, Off
1+2+4 = 7
1, On
1, On
1, On
0, Off
0, Off
0, Off
4+16 = 20
0, Off
0, Off
1, On
0, Off
1, On
0, Off
1+2+16+32
= 51
1, On
1, On
0, Off
0, Off
1, On
1, On
For example, if the DIP switch address is set to 2, then ADDR+0 = 2, so the address of the primary
SMB will be 2, the address of the next SMB will 3, and so on.
