Modbus rtu message format, Read coil status command (01) – Flowserve L75 Series Electric Actuator User Manual
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L75 Series Electric Actuator — Modbus RTU FCD LMAIM7502-00 – 07/05
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Modbus functions are seen as commands to the positioner. Hereafter in this manual, they will be called
commands. A summary of commands to cause specific valve movement are shown in the table below:
Action
Modbus Function (code)
Coil/Reg. # (address) Data
CW
Force single coil (05)
Coil 1 (0000H)
FF00 (hex)
Stop CW
Force single coil (05)
Coil 1 (0000H)
0000 (hex)
CCW
Force single coil (05)
Coil 2 (0001H)
FF00 (hex)
Stop CCW
Force single coil (05)
Coil 2 (0001H)
0000 (hex)
CW
Preset single reg. (06)
Register 40,001
256 (dec)
Stop
Preset single reg. (06)
Register 40,001
512 (dec)
CCW
Preset single reg. (06)
Register 40,001
768 (dec)
Modbus RTU Message Format
Field Name
Example
Actuator Address
05 (actuator 45)
Function
01 (Read Coil Status)
Starting Address (High)
00
Starting Address (Low)
00 (coil 1)
Number of Points (High)
00
Number of Points (Low)
01 (return 1 status)
CRC Error Check (LS byte)
FC
CRC Error Check (MS byte)
4E
NOTE: The least significant byte of the 16-bit CRC value is sent first before the most significant byte.
Each message consists of the bytes mentioned above sent consecutively with less than a 1-1/2-character
time gap between bytes. Messages are considered completed when the actuator sees a time gap greater
than 1-1/2 characters.
Read Coil Status Command (01)
This command reads the ON or OFF state of the specified coil (i.e. motion control circuit). Coil 1
represents the CW motion control circuit; coil 2 represents the CCW motion control circuit. The Modbus
message can have a starting address of 0 or 1 (for coil 1 or 2, respectively). The number of points can
be either 1 or 2 when coil 1 is specified and should be 1 if coil 2 is specified. Requesting more than
2 points with coil 1 or more than 1 point with coil 2 will result in an error response. Regardless the
number of points requested, only one data byte will be returned. Broadcast is not supported.
Example: “Read the status of the CW motion control circuit (coil 1) on actuator #5”
Field Name
Example
Actuator Address
05
Function (Read Coil Status)
01
Starting Address (High)
00
Starting Address (Low)
00
Number of Points (High)
00
Number of Points (Low)
01
CRC Error Check (LS byte)
FC
CRC Error Check (MS byte)
4E
The bytes sent (in hex) to the actuator in the above example would be: 05, 01, 00, 00, 00, 01, FC, 4E