Minimum And Maximum Values; Modbus Data Table; Troubleshooting; Preferred Method Of Testing - Hitachi Abb Rvt Installation Und Betriebsanleitung

6.2. Minimum and maximum values

Variables have a limited range. If a write operation to a variable exceeds the minimum and maximum allowable val-
ues, the variable will be overridden with its minimum or maximum value.
An ILLEGAL DATA VALUE exception error is sent back.
See the Modbus data table for more details.

6.3. Modbus Data table

Data are arranged in several tables for convenience. Individual tables contain similar information. Table data may be
read only or read/write access.
Data in each table is pointed to in a Modbus command by two consecutive data address bytes. The first byte defines
the table number, and the second byte the offset of the data in the table. These two bytes are what is called the 'Mod-
bus address' or the 'Modbus register'.
A specific Modbus data table is dedicated to a specific product type.
Access (read or write) to a non-referenced Modbus address result in an ILLEGAL DATA ADDRESS exception error.
The Modbus Data table gives all information on the various data and how to access them.
To handle RVT data, please refer to the RVT Modbus Data table.

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 antici-
pated.
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.
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