GE Multilin 565 Instruction Manual Download

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FEATURE INFORMATION

6-7

Error Checking

The RTU version of Modbus includes a two byte CRC-16 (16
bit cyclic redundancy check) with every transmission. The
CRC-16 algorithm essentially treats the entire data stream
(data bits only; start, stop and parity ignored) as one continu-
ous binary number. This number is first shifted left 16 bits and
then divided by a characteristic polynomial
(11000000000000101B). The 16 bit remainder of the division
is appended to the end of the transmission, MSByte first. The
resulting message including CRC, when divided by the same
polynomial at the receiver will give a zero remainder if no
transmission errors have occurred.

If a 565 Feeder Management Relay Modbus slave device
receives a transmission in which an error is indicated by the
CRC-16 calculation, the slave device will not respond to the
transmission. A CRC-16 error indicates than one or more
bytes of the transmission were received incorrectly and thus
the entire transmission should be ignored in order to avoid
the slave device performing any incorrect operation.

The CRC-16 calculation is an industry standard method
used for error detection. An algorithm is included here to
assist programmers in situations where no standard CRC-16
calculation routines are available.

CRC-16 Algorithm
Once the following algorithm is complete, the working regis-
ter “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 polyno-
mial is dropped since it does not affect the value of the
remainder. The following symbols are used in the algorithm:
—>

data transfer

16 bit working register

low order byte of A

high order byte of A

CRC

16 bit CRC-16 value

i,j

loop counters

(+)

logical “exclusive or” operator

i-th data byte (i = 0 to N-1)

16 bit characteristic polynomial =
1010000000000001 with MSbit dropped and bit
order reversed

shr(x) shift right (the LSbit of the low order byte of x shifts

into a carry flag, a ‘0’ is shifted into the MSbit of the
high order byte of x, all other bits shift right one
location

algorithm:
1. FFFF hex —> A
2. 0 —> i
3. 0 —> j
4. Di (+) AL —> AL
5. j+1 —> j
6. shr(A)
7. is there a carry? No: go to 8.

Yes: G (+) A —> A

8. is j = 8? No: go to 5.

Yes: go to 9.

9. i+1 —> i
10. is i = N? No: go to 3.

Yes: go to 11.

11. A —> CRC

Timing

Data packet synchronization is maintained by timing con-
straints. The receiving device must measure the time be-
tween the reception of characters. If three and one half
character times elapse without a new character or comple-
tion of the packet, then the communication link must be reset
(ie. 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.

Explanation of 565 Supported Functions

The following functions are supported by the 565 Feeder
Management Relay:
03 - Read Holding Registers (Read Setpoints)
04 - Read Input registers (Read Actual Values)
05 - Force Single Coil (Execute Operation)
06 - Preset Single Register (Store Single Setpoint)
07 - Read Exception Status (Read Device Status)
16 - Preset Multiple Registers (Store Multiple Setpoints)

FUNCTION CODE 03
Modbus implementation: Read Holding Registers
565 Feeder Management Relay implementation: Read Set-
points
This function code allows the master to read a group of
setpoints from a slave device. The maximum number of
holding registers that can be read in one transmission is 125.
Number of holding registers should match the total length of
required setpoints. In other words, reading half of a setpoint
is not recommended. For the 565 Feeder Management
Relay implementation of Modbus, “holding registers” are
equivalent to 565 Feeder Management Relay setpoints. Hold-
ing registers are 16 bit (two byte) values transmitted high
order byte first. But all 565 Feeder Management Relay
setpoints are not made of two bytes. Some of them are four
bytes long and in that case two consecutive holding register
addresses are used to transmit data of one setpoint. Holding
register with lower address number carries two more signifi-
cant bytes.

The slave response to this function code is the slave ad-
dress, function code, a count of the number of data bytes to
follow, the data itself and the CRC. Each data item (setpoint)
is sent as a two byte number with the high order byte sent
first.

Note: Broadcast mode is not allowed with this function code.
The master transmission will be ignored by all slaves if
broadcast mode is used with this function code.