Troubleshooting Guide
Checking Modbus RTU, RS485 communication
SKF Multilog On-Line System IMx-S 5 - 9
User Manual – Revision S
The Modbus RTU protocol exchanges binary data across the bus. Detection of message
start and message end rely on certain bit state combinations and inter-message
minimum pauses, with no communications traffic. For each device on the bus to
correctly interpret these from the electrical patterns on the bus requires an equal
understanding of the bit allocations and the bit rate or baud rate, being used. Commonly
an arrangement known as 8-N-1 (eight data bits, no parity bit and one stop bit)
transmitted at 9 600 or 19 200 bits per second are used. If devices are differently set, by
configuration or otherwise, communications will fail.
The Modbus RTU timing requirements also apply to responses sent when a slave device
is answering a master device command: the response must not start until a time
corresponding to 3.5 characters at the configured baud rate, has elapsed since the end
of the command. SKF has identified that some Modbus transducers do not always
adhere to these requirements and consequently a reply sent without observing the
required 3.5 character gap risks not being recognised as a valid message. In this
eventuality the IMx may be unable to successfully communicate with these Modbus
slaves and unread and input ignored error messages, time-out and frame errors will
result.
Note that although the 3.5 character gap is a hard requirement that should be observed
by all Modbus RTU equipment, it is not an absolute cut-off because thee is a illegal
timing area between 1.5 and 3.5 characters to allow inter character and inter message
(frame) gaps to be reliably distinguished. Be aware that as a consequence non-compliant
equipment may work in some test scenarios, such as with a PC master device, but not in
others such as with an IMx master device.
If difficulty is experienced with a particular type of slave and this issue is suspected,
where possible, it may be advantageous to increase the baud to 19 200 as at this higher
rate any fixed response time will correspond to twice the character gap compared to
9 600 baud.
For further information on RS485 and Modbus RTU, refer to the documents listed in the
RS485 Communication section.
When there is access to the IMx-S RS232 service interface, some further diagnostic
checks are possible:
1. Start the @ptitude Observer, On-line Device Configurator.
2. Click Start serial interface.
3. On the Serial interface dialog, select the COM port number and type in the
word "modbus" in the command box.
4. Statistics on communication errors and then summaries for each Modbus
connection will appear on the screen.
The statistics include:
- Frame errors, short and long, and checksum errors
Then for each connection the:
- Number of messages sent (tx) and received (rx)
- Number of request timeouts (to) and exceptions
Each connection has an identifier, example RM1, that uses the following key:
- T/R: TCP or RTU
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