Emerson OpenComms-485 - Manual (Page 6)

REV. 5 7/6/2007
Liebert Monitoring Group –Development
3
16 Write multiple
Registers
Write values into a block of contiguous registers (1 to
120)
Data Field(s):
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.
Error Check Field:
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
Address
Function
Code
Starting
Register
“Hi Byte”
Starting
Register
“Lo Byte”
Number of
Registers
“Hi Byte”
Number of
Registers
“Lo Byte”
CRC16
“Hi Byte”
CRC16
“Lo Byte”
02 03 00 32 00 03 E5 FA
Response Sample
Slave
Address
Function
Code
Count:
Bytes of
Data
Register
40051 Data
Hi Lo
Register
40052 Data
Hi Lo
Register
40053 Data
Hi Lo
CRC16
“Hi Byte
CRC16
“Lo Byte”
02 03 6 01 58 00 FA 00 54 1B 0D
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)
40052 = FA (hex) = 250 (decimal)
40053 = 54 (hex) = 84 (decimal)