GE 1601-0149-A2 manual Modbus Functions

Modbus Functions			489
			Communications Guide
CRC-16 Algorithm
	Once the following algorithm is complete, the working register “A” will contain the
	CRC value to be transmitted. Note that this algorithm requires the characteristic
	polynomial to be reverse bit ordered. The MSbit of the characteristic polynomial is
	dropped since it does not affect the value of the remainder.
	The symbols used in the algorithm are shown below:
	-->	data transfer
	A; Alow; Ahigh	16-bit working register; low and high order bytes of A (the 16-bit
		working register)
	CRC	16 bit CRC-16 result
	i, j	loop counters
	(+)	logical EXCLUSIVE-OR operator
	N	total number of data bytes
	Di	i-th data byte (i = 0 to N – 1)
	G	16 bit characteristic polynomial = 1010000000000001 (binary)
		with MSbit dropped and bit order reversed
	shr (x)	right shift operator (the LSbit of x is shifted into a carry flag, a '0' is
		shifted into the MSbit of x, all other bits are shifted right one
		location)

The CRC algorithm is shown below:

1.FFFF (hex) --> A

2.0 --> i

3.0 --> j

4.Di (+) Alow --> Alow

5.j + 1 --> j

6.shr (A)

7.Is there a carry? No: go to step 8.

8.	Is j = 8?			Yes: G	(+) A --> A			and continue.
				No: go	to	5.;	Yes: continue.
9.	i	+ 1	--> i	No: go	to	3.;	Yes: continue.
10.	Is i = N?
11.	A	-->	CRC

Timing Data packet synchronization is maintained by timing constraints. The receiving device must measure the time between the reception of characters. If three and one half character times elapse without a new character or completion of the packet, then the communication link must be reset (i.e. all slaves start listening for a new transmission from the master). Thus at 9600 baud a delay of greater than 3.5 ⋅ 1 / 9600 ⋅ 10 = 3.65 ms will cause the communication link to be reset.

Modbus Functions

Supported Functions The following functions are supported by the 489:

•Function Codes 03 and 04: Read Setpoints and Actual Values

•Function Code 05: Execute Operation

•Function Code 06: Store Single Setpoint

•Function Code 07: Read Device Status

•Function Code 08: Loopback Test

•Function Code 16: Store Multiple Setpoints

A detailed explanation of how the 489 implements these function codes is shown in the following sections.

GE Multilin

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