Modbus Baud Rate Analysis; Table 5-85. Character Transfer Time Vs Baud Rate; Figure 5-75. Example Communication Timing Topology - ABB TPU2000 Technical Manual

TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide

Modbus Baud Rate Analysis

If Modbus ASCII Protocol is re-examined, the Modbus frame is illustrated in Figure 5-4. The frame is a standard
10 bit frame. One character (7 data bits) is transmitted as 10 bits per frame.
The rate of the data transfer is determined by the selected baud rate. The faster the baud rate, the faster the
communication. The TPU2000/TPU2000R supports baud rates of 1200, 2400, 4800, 9600 and 19200. The effect
of transfer time is shown in Table 5-85. Each bit has specific transfer time which correlates to a specific character
transfer time.

Table 5-85. Character Transfer Time vs Baud Rate

Baud Rate Transfer Time Per Bit
300
1200 0.833 mS
2400 0.4167 mS
4800 0.2083 mS
9600 0.1041 mS
19200 0.0521 mS
These are fixed times determined by the laws of physics, and are standard for asynchronous bit stream transfers
ASCII.
Each Modbus transfer varies in the amount of bytes transmitted and requested. Table 5-86 lists the amount of
fixed data per some of the common Modbus Commands. For example, each data transmission contains the
following characters as per Figure 5-3:
• Colon (:)[ 3A Hex]
•
Slave Address (Two Characters)
•
Function (Two Characters)
•
Error Check (Two Characters)
•
Line Feed (One Character)
SCADA Master

Figure 5-75. Example Communication Timing Topology

Transfer Time Per Character
3.33 mS
Point to Point
Address X
Network
Partner
V1.0
Protective Relay
Slave Device
3.33 mS
8.333 mS
4.167 mS
2.083 mS
1.041 mS
0.521 mS
C
E
250