User`s guide

Copyright © 2004 EIM COMPANY, INC. • 13840 PIKE ROAD • MISSOURI CITY, TX. 77489
Page 117
Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)
6.5. App Note: Modbus Message Formats
All Modbus communication timing estimates in this document are based on 9600 baud.
There is always a 10ms minimum “Command Message Turn Around Time” between when the last
message was received by the 1746-C and when it can transmit again … the physical network actually
requires 8ms but 10ms is what the 1746-C delivers.
There is always a 8ms minimum “Response Message Turn Around Time” between when the last
message was received by the actuator and when it can transmit again.
At 9600 baud, it takes about 1.145ms / character transmitted … per byte in the message being
transmitted.
(11 bits in each char) / ( 9600 baud)
DIAGRAM OF COMMUNICATION ON THE NETWORK
6.5.1. Modbus RTU Functions (Command Codes) Implemented
1746-C system utilizes a subset of Modbus RTU protocol. The protocol is implemented in accordance
with Modicon Modbus Protocol Reference Guide PI-MBUS-300 Rev. J.
The following Modbus function codes are used:
Code Meaning Action and Data Type
01
Read Coil Status
Read current status of a group of discrete output bits
03 Read Holding Register Read binary value in one or more 16-bit registers
05 Set (Force) Single Coil Write a single discrete output bit on or off
06 Set Single Register Write a binary value to a specific 16-bit register
15 Set (Force) Multiple Coils Write on/off state to a group of coils
Command Message
Transmit
Response Message
Transmit
Command Message
Transmit
Turn Around
Time
(
8ms
)
Turn Around
Time
(
10ms
)
Turn Around
Time
(
8ms
)