Appendix C Modbus Commands; Modbus Protocol Overview; The Rtu Transmission Frame; Rtu Transmission Frame - Siemens SIMATIC TI500 User Manual

C.1

Modbus Protocol Overview

The RTU
Transmission Frame
C-2 Modbus Commands
This appendix describes the Modbus protocol in detail. For each of the
Modbus functions described, examples of requests and responses are given.
Table C-1 describes the Modbus functions supported by the Gateway.
Table C-1 Modbus Functions Supported
Code Type
01 Read Coil Status
02 Read Input Status
03 Read Holding Register
04 Read Input Register
05 Force a Single Coil
06 Write a Single Register
07 Read Exception Status
08 Execute Diagnostics
11 Get Communications
Event Counter
12 Get Communications
Event Log
15 Write Multiple Coils
16 Write Multiple Registers
The TIWAY I Gateway operates in the RTU transmission mode. The RTU
mode determines the start and finish of a message based on a time period of
silence (no transmission) equivalent to the time it would take to transmit
3.5 characters at the chosen baud rate. The RTU transmission frame is
shown in Figure C-1.
Address Function
8 bits 8 bits
T1 T2 T3 . . . represents a time of 3.5 characters at given baud rate.
Figure C-1 RTU Transmission Frame
Description
Get current status of a group of coils.
Get current status of a group of discrete
inputs.
Get current values from holding registers.
Get current values from input registers.
Change the state of a logic coil to On or
Off, forced On or forced Off, or unforced.
Write a value into a holding register.
Get the 8 internal status coil values.
Send diagnostic tests to a slave.
Enable the success or failure of a query to
be determined.
Get the communications log for Modbus
network transactions.
Change a number of consecutive coils.
Write values into a series of consecutive
holding registers.
Data Checksum
n x 8 bits 16 bits
TIWAY I Gateway User Manual
No data
T1 T2 T3
I000000