GIC 151A12B, Инструкция по эксплуатации

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20.0 MODBUS:
Pr-69 has adopted widely used MODBUS RTU protocol. The MODBUS RTU communication
functions implemented in PR69 series are: function 3–Read holding variables (read);
function 6-preset one word (write). These functions allow the supervisory program to
read and modify any data of the controller.
The communication is based on messages
sent by the master station (host) to the slave stations (PR69) and vice versa. Every a
message contains four fields:
a) Slave address (from 1 to 99)
b) Function code: contains 3 or 6 for specified functions.
c) Information field: contains data like word addresses and word values as required by
function in use.
d) Control word: a cyclic redundancy check (CRC) performed with particular rules for
CRC.
3.1 Function 3 - read n word
The request has the following frame:
Slave
Number
Byte 0
3 Byte 1
First Word MSB
Address LSB
Byte 2
Byte 3
Number of Words
MSB
LSB
Byte 4
Byte 5
CRC
LSB
MSB
Byte 6
Byte 7
The normal reply (as opposed to exception reply) has the following frame.
Slave
Number Byte 0
3 Byte 1
NB Number of
Read bytes
Byte 2
Value of first word
MSB
LSB
Byte 3
Byte 4
Following Word Byte 4
CRC
LSB
MSB 2
3