MODBUS Protocol
Function Codes
C-4 48i Instruction Manual Thermo Electron Corporation
The outputs in the response message are packed as one per bit of the data
field. Status is indicated as 1 = Active (on) and 0 = Inactive (off). The LSB
of the first data byte contains the output addressed in the query. The other
outputs follow toward the high order end of this byte, and from low order to
high order in subsequent bytes. If the returned output quantity is not a
multiple of eight, the remaining bits in the final data byte will be padded
with zeros (toward the high order end of the byte). The Byte Count field
specifies the quantity of complete bytes of data.
Note The values reported may not reflect the state of the actual relays in the
instrument, as the user may program these outputs for either active closed or
open.
*N = Quantity of Outputs / 8, if the remainder not equal to zero, then N=N+1
Here is an example of a request and response to read outputs 2–15:
Request
Function code 1 Byte 0x01 or 0x02
Starting Address 2 Bytes 0x0000 to maximum allowed by instrument
Quantity of outputs 2 Bytes 1 to maximum allowed by instrument
Unit Identifier 1 Byte 0x00 to 0xFF (Passed back in response)
Response
Function code 1 Byte 0x01 or 0x02
Byte count 1 Byte N*
Output Status n Byte
n = N or N+1
Error Response
Function code 1 Byte Function code + 0x80
Exception code 1 Byte 01=Illegal Function, 02=Illegal Address,
03=Illegal Data, 04=Slave Device Failure
Request
Field Name (Hex)
Function 0x01
Starting Address Hi 0x00
Starting Address Lo 0x02
Quantity of Outputs Hi 0x00
Quantity of Outputs Lo 0x0D
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