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This is an overview of the MODBUS commands that are used by the RTCU device in master mode and accepted by the device in slave mode.
Master
The MODBUS commands that the RTCU device uses to read and write I/O in remote devices.
Command
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Name
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Description
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02 (0x02)
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Reads discrete inputs.
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This is used to read the digital inputs from the devices.
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04 (0x04)
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Reads input registers.
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This is used to read the analog inputs from the devices.
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15 (0x0F)
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Writes multiple coils.
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This is used to set the digital outputs for the devices.
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16 (0x10)
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Writes multiple holding registers.
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This is used to set the analog outputs for the devices.
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Slave
The MODBUS commands that the RTCU device accepts in slave mode.
The RTCU device will return an error message with exception code 01 (0x01) if it receives a MODBUS command not listed here.
Please note that memory registers in the RTCU device are 32 bits long and are read and written lower half first and then upper half.
Register channel 0x1000 will therefore contain RTCU memory position 1 Byte 1 + 2 and channel 0x1001 will contain position 1 Byte 3 + 4.
The command tables follow this convention:
Field #
|
Name
|
Byte count
|
Value / Comments
|
01 (0x01) Read coils
Request
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x01.
|
02 - 03
|
Starting channel
|
2 bytes
|
0x0000 - 0x003F for DO onboard RTCU.
0x0040 - 0x023F for DO in extension modules.
|
04 - 05
|
Number of channels
|
2 bytes
|
0x0001 - 0x0240.
|
Error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x81.
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02
|
Exception code
|
1 byte
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0x03 for number of channels too high.
0x02 for when one or more of the channels are not present.
|
02 (0x02) Read discrete inputs
Request
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x02.
|
02 - 03
|
Starting channel
|
2 bytes
|
0x0000 - 0x003F for DI onboard RTCU.
0x0040 - 0x023F for DI in extension modules.
|
04 - 05
|
Number of channels
|
2 bytes
|
0x0001 - 0x0240.
|
Error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x82.
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02
|
Exception code
|
1 byte
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0x03 for number of channels too high.
0x02 for when one or more of the channels are not present.
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03 (0x03) Read holding registers
Request
00
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Address
|
1 byte
|
1 - 247.
|
01
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Function code
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1 byte
|
0x03.
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02 - 03
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Starting channel
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2 bytes
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0x0000 - 0x003F for AO onboard RTCU.
0x0040 - 0x013F for AO in extension modules.
0x1000 - 0x17FF for RTCU memory registers.
|
04 - 05
|
Number of channels
|
2 bytes
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0x0001 - 0x007D.
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Error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x83.
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02
|
Exception code
|
1 byte
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0x03 for number of channels too high.
0x02 for when one or more of the channels are not present.
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04 (0x04) Read input registers
Request
00
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Address
|
1 byte
|
1 - 247.
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01
|
Function code
|
1 byte
|
0x04.
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02 - 03
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Starting channel
|
2 bytes
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0x0000 - 0x003F for AI onboard RTCU.
0x0040 - 0x013F for AI in extension modules.
0x1000 - 0x17FF for RTCU memory registers.
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04 - 05
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Number of channels
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2 bytes
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0x0001 - 0x007D.
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error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x84.
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02
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Exception code
|
1 byte
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0x03 for number of channels too high.
0x02 for when one or more of the channels are not present.
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05 (0x05) Write single coil
Request
00
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Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x05.
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02 - 03
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Channel
|
2 bytes
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0x0000 - 0x003F for DO onboard RTCU.
0x0040 - 0x013F for DO in extension modules.
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04 - 05
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Value
|
2 bytes
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0x0000 or 0xFF00.
|
error response
00
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Address
|
1 byte
|
1 - 247.
|
01
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Function code
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1 byte
|
0x85.
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02
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Exception code
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1 byte
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0x03 for invalid value.
0x02 for invalid channel.
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06 (0x06) Write single holding register
Request
00
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Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x06.
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02 - 03
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Channel
|
2 bytes
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0x0000 - 0x003F for AO onboard RTCU.
0x0040 - 0x013F for AO in extension modules.
0x1000 - 0x17FF for RTCU memory registers.
|
04 - 05
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Value
|
2 bytes
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0x0000 - 0x03FF for AO onboard RTCU.
0x0000 - 0xFFFF for all other channels.
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Error response
00
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Address
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1 byte
|
1 - 247.
|
01
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Function code
|
1 byte
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0x83.
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02
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Exception code
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1 byte
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0x02 for invalid channel.
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15 (0x0F) Write multiple coils
Request
00
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Address
|
1 byte
|
1 - 247.
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01
|
Function code
|
1 byte
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0x0F.
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02 - 03
|
Starting channel
|
2 bytes
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0x0000 - 0x003F for DO onboard RTCU.
0x0040 - 0x023F for DO in extension modules.
|
04 - 05
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Number of channels
|
2 bytes
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0x0001 - 0x0240.
|
06
|
Byte count
|
1 byte
|
N.
|
07 -
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Output values
|
N * 1 byte
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Value.
|
error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x8F.
|
02
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Exception code
|
1 byte
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0x03 for number of channels too high or with invalid byte count
0x02 for when one or more of the channels are not present.
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16 (0x10) Write multiple holding registers
Request
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x10.
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02 - 03
|
Starting channel
|
2 bytes
|
0x0000 - 0x003F for AO onboard RTCU.
0x0040 - 0x013F for AO in extension modules.
0x1000 - 0x17FF for RTCU memory registers.
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04 - 05
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Number of channels
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2 bytes
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0x0001 - 0x007B.
|
06
|
Byte count
|
1 byte
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2 * N.
|
07 -
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Values
|
N * 2 bytes
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Value.
|
Error response
00
|
Address
|
1 byte
|
1 - 247.
|
01
|
Function code
|
1 byte
|
0x90.
|
02
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Exception code
|
1 byte
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0x03 for number of channels too high of invalid byte count
0x02 for when one or more of the channels are not present
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