Communication bus wiring or termination fault

ID: fm:communication-bus-fault
IRI: https://solarlattice.org/kb/fm/communication-bus-fault
Status: active
Also known as: missing terminating resistor, terminating resistor missing, missing terminator, RS485-all-fail, signal reflections, RJ45 crimp fault, cross-wired signal wiring, terminator not inserted, RS485 AllFail

On an RS485 line with no termination, signals reflect back from the open ends, and the terminator absorbs them when it matches the cable impedance. The fault shows up as lost communication with some or all inverters, a master, or a Multicluster Box, or as faulty CAN communication with a missing internal communication pulse. Poor contact at the terminal blocks of internal communication wires can also raise a communication error alarm. To confirm it, test the terminating resistor and check the termination and wiring of the communication buses. Fix it by fitting a terminator at both ends of the bus or turning on the termination switch on the end devices, and by using undamaged, correctly plugged CAT5e cables. Termination is not always needed: at low baud rates it may be unnecessary, and it adds load to the line, so decide site by site.

<!-- review notes

sentence 4: support check: partial - Passages list testing the terminating resistor and checking communication bus termination, but give no detail on confirming the fault.
sentence 4: review: partial - The passages are only headings and checklist items for testing the terminating resistor and checking bus termination and wiring; nothing presents them as steps to confirm this fault. -->

Claims

`caused_by`

c1 drafted · lead Installation error

A terminating resistor that is missing or wrong at a bus end node disturbs data transmission.

Enphase troubleshooting Energy Systems with IQ Battery 5P sec=h48 h=21e245e5e474
275752 sec=h162 h=398aa158ed89

c2 drafted · lead Poor connector contact

CommErr alarm is attributed to poor contact at the terminal block connectors of the internal communication wires.

CPS 36kW usermanual reduced tbl=37.r5 h=ba3a944e3c4f
CPS SCH100-125KTL-DO-US-600-480 User Manual Rev 1.9 tbl=32.r2 h=8a49e69e6164

`affects`

c3 drafted · lead Communication cable proposed

Enphase troubleshooting Energy Systems with IQ Battery 5P sec=h48 h=21e245e5e474
278742 tbl=73.r6 h=e21d8d120311
259761 tbl=69.r6 h=62365a359481

c4 drafted · lead Internal communication cable

CPS 36kW usermanual reduced tbl=37.r5 h=ba3a944e3c4f
CPS SCH100-125KTL-DO-US-600-480 User Manual Rev 1.9 tbl=32.r2 h=8a49e69e6164

`presents_as`

c5 drafted · lead Communication loss

Examples include loss of communication with one or all inverters, with a Multicluster Box, or with the master device.

Solis_Manual_EPM3_5G_Pro_20250724 sec=h109 h=7de92bdd095c
259761 tbl=69.r6 h=62365a359481
SI30M-44M-60H-80H-BE-en-33W tbl=68.r8 h=e3fb0c52c01d
259761 sec=h818 h=c02dfb4d0c55

c6 drafted · lead Internal communication failure

278742 tbl=73.r6 h=e21d8d120311
259761 tbl=69.r7 h=c63c963a8783
CPS 36kW usermanual reduced tbl=37.r5 h=ba3a944e3c4f
CPS SCH100-125KTL-DO-US-600-480 User Manual Rev 1.9 tbl=32.r2 h=8a49e69e6164

c7 drafted · lead Missing synchronization pulse

The device-internal communication pulse is missing on at least one device in the cluster.

278742 tbl=73.r6 h=e21d8d120311
259761 tbl=69.r7 h=c63c963a8783

`detected_by`

c8 drafted · lead Communication bus termination resistance check

Enphase troubleshooting Energy Systems with IQ Battery 5P sec=h47 h=9ef4538d56bd
259761 sec=h309 h=255d449d5cb8
278742 sec=h322 h=acd141cd00e5

review note: support check: partial - Passages mention testing the terminating resistor and checking bus termination, but don't state it detects the fault or describe a resistance check.

`mitigated_by`

c9 drafted · lead Communication bus termination and cabling practice

Fit a terminator at both ends of the bus, for example 120 ohm between CAN_H and CAN_L, or set the RS485 termination switch to ON on end devices.

275752 sec=h162 h=398aa158ed89
278742 tbl=73.r6 h=e21d8d120311
278742 sec=h893 h=759302c4d7ce
259761 sec=h779 h=759302c4d7ce
278742 sec=h909 h=759302c4d7ce
259761 sec=h793 h=759302c4d7ce
259761 tbl=69.r6 h=62365a359481
259761 tbl=69.r7 h=c63c963a8783
SI30M-44M-60H-80H-BE-en-33W tbl=68.r8 h=e3fb0c52c01d
259761 tbl=72.r7 h=76e35eccb4ba
278303 sec=h83 h=79df67d51bdf
CPS SCH100-125KTL-DO-US-600-480 User Manual Rev 1.9 tbl=16 h=659fb2d59198
CPS SCH100-125KTL-DO-US-600-480 User Manual Rev 1.9 tbl=16.r2 h=05240a992756

review note: support check: partial - Terminator at both ends and the RS485 termination switch ON are supported, but 120 ohm between CAN_H and CAN_L is only via pin descriptions; broadly supported though RS485 'end devices' is not stated.

c10 verified · fact Communication cable check

Use undamaged CAT5e cables and make sure they are plugged in correctly.