IAI America REXT - Manual (Page 145)

Part 1 Specification
Chapter 3 Gateway R unit
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Part 1 Specification
(4) Message Frame
Query and response messages use the following message frame.
Header Address Function Data Error check Trailer
T1-T2-T3-T4 8 bits 8 bits N x 8 bits 16 bits T1-T2-T3-T4
* “T1-T2-T3-T4” represents a silent interval.
[1] Header field
The frame starts with a silent interval of 3.5 characters or more.
[2] Address field
The address in the message frame. Fixed to 3FH for the RS485 SIO gateway.
[3] Function field
The following three functions can be used in the Modbus gateway mode.
Code (Hex) Name Function
Broadcast
support
03H Read Holding Registers Read holding registers
06H Preset Single Register Write holding register {
10H Preset Multiple Registers Write multiple holding registers simultaneously {
[4] Data field
This field is used to append data relating to the function code. The data field has a variable length and it can be
omitted.
[5] Error check field
16-bit data is indicated by two bytes of eight-bit data.
The error check result is calculated by the method called “CRC (Cyclical Redundancy Check).”
[6] Trailer field
The frame ends with a silent interval of 3.5 characters or more.
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