Emerson OpenComms-485 manual RTU Framing

16

	REV. 5 7/6/2007
	Liebert Monitoring Group –Development
Write multiple	Write values into a block of contiguous registers (1 to
Registers	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	Function	Starting	Starting
Address	Code	Register	Register
		“Hi Byte”	“Lo Byte”
02	03	00	32

Response Sample

Number of	Number of	CRC16	CRC16
Registers	Registers
“Hi Byte”	“Lo Byte”	“Hi Byte”	“Lo Byte”
00
	03	E5	FA

Slave

Address

Function

Code

Count:

Bytes of

Data

Register Register Register

40051 Data 40052 Data 40053 Data

Hi Lo Hi Lo 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)

3