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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:
Enter the MODBUS menu under:
COMMUNICATION - I/O CONFIGURATION – COM. SETTINGS – MODBUS
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Choose a slave address
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Select the right communication speed (baud rate)
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Select the right parity checking
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Select the right number of stop bits
16 – Troubleshooting ç RS-485
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