Modbus Serial Diagnostic Counters; Troubleshooting Serial Communication - ABB Relion 620 Series Protocol Manual

1MRS757645 B
3.2.1.2
3.2.1.3
620 series
Communication Protocol Manual

Modbus serial diagnostic counters

Modbus Serial diagnostic counters can be viewed via the LHMI path Monitoring/
Communication/Modbus/MBS0n.
The counters show complete Modbus protocol link frames and Modbus errors. The
serial communication drivers (COM1, COM2) maintain their own counters for lower
level serial communication diagnostics.
Table 2: Serial diagnostic counters
Counter Description
Status Shows the value "True" if the serial instance is in use. This indicates that the
Modbus client is connected and Modbus messages, which are addressed to
the device, are received regularly at least with a 15 second interval or faster. In
all other cases this value is "False".
Reset counters True = Reset all diagnostic counters
Received frames Total amount of received Modbus frames. For example, the Modbus frames
that are addressed to this instance.
Transmitted frames Total amount of transmitted Modbus responses.
Transmitted exc A Total amount of exception responses 1 and 2.
These exception responses usually reveal configuration errors on the Modbus
client side. Either the client uses a request function code which is not supported
or the requested Modbus point(s) does not exist.
Transmitted exc B Total amount of exception responses 3.
These exceptions usually reveal the IED application level rejections. That is,
the IED application rejects the request at this moment, under the current
circumstances. The exception can also mean that the value in the Modbus
write request is out of range.
Checksum errors Total amount of detected Modbus checksum errors.
The Modbus instance only calculates checksums of Modbus frames that
contain a proper link address. All other incoming Modbus frames are
discarded.

Troubleshooting serial communication

The diagnostic capabilities can be used for investigating communication problems. If
communication cannot be established to the relay, then proceed in this order.
1. Reset the serial driver and Modbus protocol diagnostic counters to make it easier
to view the changes.
2. Check the serial driver diagnostic counters. If serial characters are not received,
check the cable (Rx line) and the link setup parameters, also on the Master
side.
3. If serial characters are received, check if whole link frames are also received. Do
this first on the driver side.
4. Go over to Modbus diagnostics and check if Modbus link frames are internally
received. Note that the serial driver detects every link frame on the line, but the
Modbus protocol in turn only reacts to link frames of Modbus type, which are
addressed to its own protocol instance.
Vendor-specific implementation
Section 3
17