Chapter 6 Communication Mode
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6.3
Modbus master mode
In modbus master mode, EH-SIO is able to send requests (called “query” in this manual) according to the modbus
protocol to an external device (a maximum of 32 sets), and receives an answer (called “response” in this manual) from
external device by TRNS 9 command.
A connected device works according to function code, which is included in a query.
6.3.1
Overview
Upon completion of initial setting, EH-SIO will be waiting for
query
transmission. Query is sent by TRNS 9
command. When the parameter for sending data area is stored and TRNS 9 command is executed, EH-SIO will
generate query and will send it. When the reply comes, EH-SIO stores data in the receiving data area of CPU
module according to the format.
Figure 6.22 Overview of Modbus master mode
Error detection
(1) EH-SIO hardware error
Hardware error is detected by the self-diagnostic function of EH-SIO, operation will stop if a serious failure
occurs. In this case, please reboot the power, or push the reset switch on the front of the module.
(2) Query error
An error code is stored in the return code area of TRNS 9 command, when data error is detected or when
communication errors (a parity error, abnormalities in a check code, etc.) are detected. Operation will not stop
upon these errors. Rebooting or resetting are not necessary.
(3) An exception response from the external device
Although the communication itself may be performed normally, when the message format is not correct (for
example, the specified function code is not supported), an exception response returns from slaves. When an
exception response is received, EH-SIO stores an exception response code in the receiving data area and return
code area of TRNS 9 command. Operation will not stop upon these errors. Rebooting or resetting are not
necessary.
Slave device
Command
Query
Response
Sending data area
Receiving data area
TRNS 9
Response
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