CARLO GAVAZZI EM 21 Руководство по протоколу связи - Страница 3
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Energy management
1.1
Introduction
The RS485 serial interface supports the MODBUS/JBUS (RTU) protocol. In this document only the
information necessary to read/write from/to EM21 has been reported (not all the parts of the protocol
have been implemented).
For
a
complete
"Modbus_Application_Protocol_V1_1a.pdf" document that is downloadable from the
web site.
1.2
MODBUS functions
These functions are available on EM21:
Reading of n "Holding Registers" (code 03h)
Reading of n "Input Register" (code 04h)
Writing of one "Holding Registers" (code 06h)
Diagnostic (code 08h with sub-function code 00h)
Broadcast mode (writing instruction on address 00h)
IMPORTANT:
1) In this document the "Modbus address" field is indicated in two modes:
1.1) "Modicom address": it is the "6-digit Modicom" representation with Modbus function code 04
(Read Input Registers). It is possible to read the same values with function code 03 (Read
Holding Registers) replacing the first digit ("3") with the number "4".
1.2) "Physical address": it is the "word address" value to be included in the communication
frame.
2) The functions 03h and 04h have exactly the same effect and can be used indifferently.
3) The communication parameters are to be set according to the configuration of the instrument
(refer to EM21 instruction manual)
1.2.1
Function 03h (Read Holding Registers)
This function is used to read the contents of a contiguous block of holding registers (word). The Request
frame specifies the starting register address and the number of registers to be read. It is possible to
read maximum 11 registers (words) with a single request, when not differently specified.
The register data in the response message are packed as two bytes per register (word), with the binary
contents right justified within each byte. For each register, the first byte contains the high order bits
(MSB) and the second contains the low order bits (LSB).
Request frame
Description
Physical address
Function code
Starting address
Quantity of registers (N word)
CRC
Response frame (correct action)
Description
Physical address
Function code
Quantity of requested bytes
Register value
CRC
EM24-DIN Communication Protocol
description
of
the
Length
1 byte
1 to F7h (1 to 247)
1 byte
03h
2 bytes
0000h to FFFFh
2 bytes
1 to 10h (1 to 11)
2 bytes
Length
1 byte
1 to F7h (1 to 247)
1 byte
03h
1 byte
N word * 2
N*2 bytes
2 bytes
MODBUS
protocol
Value
Byte order: MSB, LSB
Byte order: MSB, LSB
Value
Byte order: MSB, LSB
please
refer
to
www.modbus.org
Note
Note
3
the