BENSHAW SPR-100P, Manual

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Read Holding Registers Function code 03h is used to read the contents of the single Remote RTD Module
holding register. Since there is only one register available to read, using the register
offset of 00 00h will read the contents of register 40001. For example, to check the
mode of the Remote RTD Module, send the command;
Address Function
Code Register Offset Number of
Registers CRC-16
10h 03h 00 00h 00 01h 87 4Bh
If the Remote RTD Module is in the test mode for RTD#4, it will respond with the
following;
Address Function
Code Byte Count Register Data CRC-16
10h 03h 02h 00 04h 45 84h
Preset Single Register Function Code 06h allows the Modbus master to write to the holding register in the
Remote RTD Module. This command allows the mode of the Remote RTD Module to
be set. For example, to set the Remote RTD Module to test mode so that it only
monitors the temperature of RTD#7, send the command;
Address Function
Code Register Offset Register Data CRC-16
10h 06h 00 00h 00 07h CB 49h
The Remote RTD Module will respond with this echo of the command;
Address Function
Code Starting
Register Number of
Registers CRC-16
10h 06h 00 00h 00 07h CB 49h
4.5 Modbus Exception Responses
General There are four different communications errors the Remote RTD Module will
recognize and respond to. They are;
•
CRC Error
• Illegal Function (exception code 01)
• Illegal Data address (exception code 02)
•
Illegal Data Value (exception code 03)
CRC Error If the Remote RTD Module receives a message that contains a CRC error, the received
message is ignored. No response is sent.
Illegal Function If the Remote RTD Module receives a message that is anything other than read
holding registers, read input registers, or preset single register (functions 03, 04, 06),
it will respond with and illegal function exception response. This response will be in
the form;
• Byte 1: RTD Module Address.
•
Byte 2: The requested function code with the MSB set to 1 to indicate an
exception.
•
Byte 3: Exception code 01h indicating an illegal function code.
•
Byte 4: LSB of CRC code.
• Byte 5: MSB of CRC code.
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4. COMMUNICATING WITH THE REMOTE RTD MODULE