ICP DAS USA ET-7H16M Series User Manual page 80

A.1. Address
The first byte of information in the message structure of Modbus is the receiver's address. The
valid addresses are in the range of 0 to 247. Addresses from 1 to 247 are given to individual
Modbus devices and 0 is used for broadcast.
Reference Description
0xxxx Read/Write Discrete Outputs or Coils. A 0x reference address is used
to device output data to a digital output channel.
1xxxx Read Discrete Inputs. The ON/OFF status of a 1x reference address is
controlled by the corresponding digital input channel.
3xxxx Read Input Registers. A reference register contains a 16-bit number
received from an external source --- e.g. an analog signal.
4xxxx Read/Write Output or Holding Registers. A 4x register is used to store
16-bits of numerical dada (binary or decimal), or to send the data from
the CPU to an output channel.
A.2. Function Codes
The second byte in the frame structure is the function code. The function code describes what the
slave is required to do. Valid function codes are between 1 and 255. The slave uses the same
function code as the request to answer it. Only when error occurs in the system, the highest bit of
the function code will be made '1'. Hence the master will know if the message has been
transmitted correctly or not.
Code
01 (01H)
02 (02H)
03 (03H)
04 (04H)
05 (05H)
06 (06H)
15 (0FH)
16 (10H)
PET-7H16M Series User Manual, version 1.0.1
Function
Read Coils (Output) Status
Read Input Status
Read Holding Registers
Read Input Registers
Force Single Coil (Output)
Preset Single Register
Force Multiple Coils (Outputs)
Preset multiple Registers
Reference
0xxxx
1xxxx
4xxxx
3xxxx
0xxxx
4xxxx
0xxxx
4xxxx
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