Troubleshooting; Preferred Method Of Testing; Check Of Identical Slave - Master Configuration; Check The Cabling Of The Rs485 - ABB RS485 Installation And Startup Manual

7 Troubleshooting

7.1 Preferred method of testing

In most cases the Modbus Master (usually a PC software package) will be running on a
personal computer or on hardware which has RS232 communications ports. In these
instances a converter will be needed to convert the RS232 signals to the RS485 standard
used by the controller. Only when the Modbus Master is running on hardware fitted with
RS485 ports is a converter unnecessary.
The preferred method of testing communications configuration is to operate with a single
controller located close to the Master (PC, PLC etc) and minimum cable length.
Once cable connections, controller configuration and Modbus Master configuration have
been successfully tested the system may be connected to the final installation (multiple
slaves and extended cabling systems as needed to meet user requirements). Full system
configuration and testing can then take place.
The software used to provide the Modbus Master functionality is not supplied. The
commissioning engineer should be familiar with the use of the software both while testing
connection to controllers and during full system configuration. In general software used
during testing can be classed as either application software or Modbus test software, the
principal differences between these two types are as follows.
Application software (such as Data Loggers or SCADA systems) is intended to use
Modbus devices to perform a measurement, control or logging function. It often provides
tools for quick configuration of known types of instrument. Such packages make it easy
for a user to start a system without needing to know Modbus commands, but may not
provide the tools for a commissioning engineer to diagnose problems when a system
does not respond as anticipated.
Modbus test software normally allows Modbus commands to be generated as required
and the responses analyzed. Some knowledge of the Modbus protocol and the
instrument register addresses will be required before using such a package. Test
packages will assist a commissioning engineer in diagnosing communication problems.
7.2 Check of identical Slave – Master configuration
In any case the master must have the same communication parameters as the controller:


7.3 Check the cabling of the RS485



Enter the MODBUS menu under:
o
COMMUNICATION - I/O CONFIGURATION – COM. SETTINGS –
MODBUS
Choose a slave address

 Select the right communication speed (baud rate)
Select the right parity checking

 Select the right number of stop bits
The non-inverting and the inverting output wires should be cabled respectively on
non-inverting and inverting input respectively. No communication will take place if
non-inverting wires are mixed with inverting wires.
Load termination resistors are important at both end of the network to avoid
distortion due to reflections in the communication line. Check that the 'Load
switch' is at the right position (ON or OFF) according to the network configuration.
RS485  Troubleshooting - 17