Modbus Plus Troubleshooting - ABB TPU2000 Technical Manual

TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide

Modbus Plus Troubleshooting

Schneider Electric has designed Modbus Plus to be a very robust communication network. The publication 890
USE 100 00 Version 2.0 titled Modicon Modbus Plus Network Planning and Installation Guide Copyright 1995,
AEG Schneider Automation, Inc. details the method for troubleshooting a Modbus Plus network.
There is no communication analyzer for Modbus Plus to view the communication commands occurring over the
network. The SA 85 or PCMCIA Modbus Plus adapters have a software utility called MBP STAT which allows a
personal computer to attach to the network allowing a network professional to troubleshoot a Modbus Plus
anomaly. Please reference the MBP STAT documentation for use of this valuable troubleshooting tool.
Issues common to Modbus Plus communication errors arise from the following:
1. Improper Device Termination. The manufacturer's in-line and termination connectors must be used.
Termination connectors must be used at the end of the lines for a string. The end of the string could
be a repeater, bridge, repeater, or end device node.
2. Improper cable used. The manufacturer's cable should be used in that it is the correct impedance,
capacitance and physical wire dimensions to physically mate with the connector.
3. Improper addressing is assumed. The TPU2000/2000R 's address is in HEX. The Modbus Plus host
and MBP STAT uses decimal addressing. Additionally, it must be remembered that an additional
byte must be appended to the end of the address signifying the path the host wishes to communicate.
4. Improper routing of the cable. The cable should be routed clear of high current devices and wires.
Although, Modbus Plus is an industrial network, it is not recommended to route or wrap the cable
around devices (such as bus bar) which emit EMI/RFI or high current spike devices. The network is a
serial network, branches, or splits are not allowed in the cabling.
necessary, please use fiber optics.
5. Improper grounding of the cable.
If proper care is not taken in the planning and installation of the network, the time saved on planning and
installation is usually spent and exceeded in troubleshooting of the network. Since Modbus Plus is a serial
network, any loose connection, impedance mismatch, or anomaly is usually difficult to find. Cable planning and
installation errors are usually seen as communication errors or retries seen using the MBP STAT utility. The
cable must be checked for continuity (in case of damage) and usually the cable must be disconnected and the
cable sections checked.
A copy of the device troubleshooting section from the aforementioned Modbus Plus text is included for the benefit
of the reader. The section covers cable continuity.
Before checking continuity, disconnect all network cable connectors from the node devices-
leave the drop cable ground lugs (ed. Note – if any since the tap and drop connectors have
ground lugs whereas the dark and light grey connectors do not).to their site panel grounds.
At any node device connector, measure the resistance between pins 2 and 3 (the signal
pins), in the range of 60..80 Ω , which include the cable wire resistance.
At each node device connector, check for an open circuit between pin 2 (a signal pin) and pin
1 (the shield pin); then check between pin 3 (a signal pin) and pin 1 – an open circuit should
exist for both checks.
At each connector, check the continuity between pin 1 and the plant ground point on the local
site panel or frame – direct continuity should be present.
If the above continuity checks are not consistent, break the network at various points, and it is recommended that
a termination connector be selected at the break. Perform the above tests using MBP STAT and the continuity
tests outlined above until the error rate is at a negligible level.
Also as with the Modbus test procedures, Modbus Plus has access to the communication status registers.
Another method to troubleshoot the TPU2000/TPU2000R is to use Read 4X Holding Register command via a
DDE editor or a PLC's MSTR instruction. The communication status registers may then be accessed. The
If such configurations are
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