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7 MODBUS PROTOCOL
7.1 Introduction to Modbus Protocol (RTU only)
Modbus communication is based on a master and a slave arrangement. The master sends a message to one slave at a time and
waits for a reply.
The slave cannot accept a new message until the existing message is processed and a reply sent to the master (maximum
response time 250 milliseconds). The slave monitors the elapsed time between receipt of characters. If the elapsed time without
1
a new character is 3
/
character times, the slave assumes the next character received is the start of a new message.
2
To allow the master to differentiate between more than one slave in a system, each slave is given a unique identity address
(between 1 and 99).
A broadcast address (address zero) can be used to access all slave devices with one command. This is limited to write
messages only and there is no slave acknowledgment.
Note. Modbus RTU requires 1 start bit, 8 data bits, 1 parity bit (optional) and 1 or 2 stop bits. The 4600 uses only
1 stop bit.
7.2 Modbus Function Codes – Table 7.1
The function code field instructs the addressed slaves which function to perform.
M
o
d
b
u
s
M
o
d
b
u
F
u
n
t c
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C
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0
1
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a
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0
3
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a
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0
5
F
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0
6
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0
8
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1
6
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Table 7.1 Modbus Function Codes
4
6
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7
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