TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide
250
SCADA Master
Protective Relay
Point to Point
Slave Device
Address X
E
C
Network
Partner
V1.0
Figure 5-75. Example Communication Timing Topology
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 Transfer Time Per Character
300 3.33 mS 3.33 mS
1200 0.833 mS 8.333 mS
2400 0.4167 mS 4.167 mS
4800 0.2083 mS 2.083 mS
9600 0.1041 mS 1.041 mS
19200 0.0521 mS 0.521 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)
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