Supported Function Codes; Function Code 3 (03H) - Read Holding Registers - Veesta World V-PM2000 Series User Manual

VP-2040 Enhanced IED
V-PM2000 Series, Veesta Power Measurement
Instructions & User's Manual
8.5

Supported Function Codes

8.5.1

Function Code 3 (03H) - Read Holding registers

8.5.1.1 Message format and example
Master
Transmission
Slave Address
Function Code
Data starting
Address
Number of
Setpoints
CRC
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The VP-2040 implementation of Modbus®, these commands can be
used to read any set-point (" holding registers ") or actual value
(" input registers ").
Holding and input registers are 16 bits (two byte) values transmitted
for high order byte first. Thus all VP-2040 set-points and actual
values are sent as two bytes.
The maximum number of registers that can be read in one
transmission is 45. Function code 03 is configured to read set-points
or actual values interchangeably because some PLCs do not support
it.
The slave response to these function codes is the slave address,
function code, a count of number of data bytes to follow the data
itself and the CRC.
Each data item is sent as a two-byte number with the high order byte
sent first.
Request slave 01 to respond with 3 registers starting at ModBus
address of 41001 (address 03E8 hex). For this example the register
data in these addresses is:
Register Number HEX Address
41001
41002
41003
Bytes
1
1
2
2
2
Chapter 8 - Protocol and Communication
03E8
03E9
03EA
Example
Meaning
(hex)
01 Message for slave 01
03 Read registers
03 Data starting at 41001 (03E8)
E8
00 3 registers (6 bytes total)
03
85 CRC calculated by the master
BB
Data (HEX)
0032
0005
0001
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