Emerson Liebert OpenComms-485 Reference Manual page 6

16 Write multiple
Registers
The data field varies in length depending on whether the message is a request or
a response to a packet. This field typically contains information required by the
slave device to perform the command specified or to pass back data to the
master device.
The Error Check Field consists of a 16-bit (2 byte) Cyclical Redundancy Check
(CRC16). It allows the receiving device to detect a packet that has been
corrupted with transmission errors.
RTU Framing
The example below shows a typical Q/R from a OC-485 interface card. In common
terms, the master device initiates a query asking slave device 2 for holding registers
starting at holding register 40051 (decimal 50) and including next 2 Registers (3 total).
Query Sample
Slave Function
Address Code
02 03
Response Sample
Slave Function
Address Code
02 03
Slave address 2 responds to Function Code 3 with 6 bytes of hexadecimal data
and ends with CRC16 checksum.
Register values:40051 = 158(hex) = 344(decimal)
Write values into a block of contiguous registers (1 to
120)
Data Field(s):
Error Check Field:
Starting Starting
Register Register
"Hi Byte" "Lo Byte"
00 32
Count: Register
Bytes of 40051 Data
Data Hi
6 01
40052 = FA (hex) = 250 (decimal)
40053 = 54 (hex) = 84 (decimal)
Liebert Monitoring Group –Development
Number of Number of
Registers Registers
"Hi Byte" "Lo Byte"
00
Register Register
40052 Data 40053 Data
Lo Hi Lo Hi
58 00 FA 00
3
7/6/2007
REV. 5
CRC16
"Hi Byte"
03 E5
CRC16
Lo "Hi Byte
54 1B
CRC16
"Lo Byte"
FA
CRC16
"Lo Byte"
0D