PMSTR-4000
Quick Start Tips
September 15, 2021
Page 4
TROUBLESHOOTING MODBUS COMMUNICATION ISSUES
Here are some common issues with Modbus wiring between the PMSTR-4000 and the
connected units:
Modbus wiring is backwards – swap the wires between A+ and B- on the
PMSTR-4000 terminal block. A+ should connect to all the terminals P12.3 (in
series) and B- should connect to all the terminals P12.4 (in series).
Modbus wiring is inconsistent – some units may have the wiring connected
backwards while others do not. This may appear as only some units
communicating.
Addresses programmed into the units are not correct. These need to be set from
1 to 8. Since these are the same addresses used from the BMS Modbus port,
ensure that the BMS integrators know that these can’t be changed. Note that by
default, a new unit has an address of 247.
One of the connected units had the LEAD/LAG key pressed on the keypad and is
attempting to become master. This will cause interference and loss of
communication on the Modbus network as a network can typically only have one
master. Follow the steps described earlier in this document to disable the
LEAD/LAG key on the keypad to prevent this. The key will highlight in green or
flash in green when it has been pressed.
If the Modbus network has a long run of wiring or a lot of connected units, it may
benefit to add a 120-ohm resistor across P12.3 and P12.4 at the last unit. This is
referred to as a “termination resistor” and can help mitigate the impedance in the
large run of wiring.
