4.3
Modbus Holding Registers
General
The Benshaw Remote RTD Module has a single Modbus holding register. This
register is used to set the operating mode of the module. In normal mode, the RTD
module scans all eight RTDs at an update rate of 1 RTD per second. In test mode, it
continually monitors the chosen RTD. The default mode at power-up is normal mode.
Holding Register
The following table list the Remote RTD Module holding register values;
Register
Description
Values
40001
Operation Mode
0 = Normal mode
1 = Test mode, RTD# 1
2 = Test mode, RTD# 2
3 = Test mode, RTD# 3
4 = Test mode, RTD# 4
5 = Test mode, RTD# 5
6 = Test mode, RTD# 6
7 = Test mode, RTD# 7
8 = Test mode, RTD# 8
4.4
Modbus Commands
General
The Remote RTD Module supports the following Modbus commands;
•
Read input registers (function code 04h).
•
Read holding registers (function code 03h).
•
Preset single register (function code 06h).
Format
All Modbus messages have the following format;
Address
Function code
Data
CRC-16
1 byte
1 byte
n-bytes
2 bytes
Read Input Registers
Function code 04h is used to read one or more consecutive input registers. The first
register read is 30001 plus the offset. For example, to read the temperature of RTD #4
(input register 30004), send the command;
Address
Function
Code
Register Offset
Number of
Registers
CRC-16
10h
04h
00 03h
00 01h
C2 8Bh
If the temperature being read by RTD#1 was 123
o
C, the Remote RTD Module will
respond with the Modbus holding register in the following format;
Address
Function
Code
Byte Count
Register Data
CRC-16
10h
04h
02h
00 7Bh
05 10h
11
4. COMMUNICATING WITH THE REMOTE RTD MODULE
