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User Guide
1
Modbus TCP Communication with a SPOT Thermometer
The SPOT thermometer functions as a Modbus slave. To communicate with a
SPOT thermometer, your device must be configured as a Modbus master.
Your device's IP address must be compatible with, but not the same as, that
of the SPOT thermometer. Setting up a compatible IP address is explained
fully in Section 2.6 of the SPOT Thermometer User Guide.
The SPOT thermometer uses standard Modbus TCP protocol, with messages
of the format:
Modbus TCP Frame Format
Name
Transaction Identifier 2 bytes For synchronisation between messages of
Protocol Identifier
Length Field
Unit Identifier
Function Code
Data Bytes
The SPOT thermometer uses read and write instructions to holding registers
for all communication. It does allow direct access to its inputs and outputs.
Thus the only Modbus Function Codes that are supported are 3 (read one or
more registers), 6 (write to a single register) and 10 (write multiple).
The full list of holding registers is given below for reference. Section 2 gives
pseudocode examples of how to implement common functions.
Length Function
server & client
2 bytes Zero for Modbus/TCP
2 bytes Number of remaining bytes in this frame
1 byte
Slave Address (255 if not used)
1 byte
Function codes (see below)
n bytes Data as response or commands
Spot Modbus Controls
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