7-4 750/760 Feeder Management Relay GE Multilin
7.3 MODBUS OPERATIONS 7 COMMUNICATIONS
7
7.3MODBUS OPERATIONS 7.3.1 MODBUS FUNCTION CODES
a) SUPPORTED FUNCTION CODES
Modbus officially defines function codes from 1 to 127 though only a small subset is generally needed. The 750/760 relays
support some of these functions, as summarized in the following table. Subsequent sections describe each function code in
detail.
b) FUNCTION CODE 01H/02H: READ BINARY STATUS
These function codes allow the master to read one or more consecutive binary status bits from an 750/760. The status bits
are packed into bytes with the first addressed bit occupying the least significant bit position of the first returned byte.
Enough bytes are returned to contain all requested status bits. The last byte is zero-padded as required. The maximum
number of status bits that can be read in a single request is 1920 (although this greatly exceeds the number of status bits
defined in the 750/760).
The addresses of the bits that can be read using these functions are the same as the point indices defined for the DNP
Binary Input objects (e.g., address zero references the “Relay In Service” status). Refer to the Device Profile Document on
page 7–49 for the definition of all binary status data. Note that function codes 01H and 02H are identical in their operation.
The following table shows the format of the master and slave packets. The example shows a master device requesting 10
status bit values starting at address 13h from slave device 11; the slave device responds with the bit values 1, 0, 1, 1, 0, 0,
1, 0, 0 and 1 from binary status addresses 13h through 1Ch inclusive. Note that two bytes are required to contain the
response data. The first byte contains the first eight data bits stored in the least significant to the most significant bit posi-
tion. The second byte contains the last two data bits stored in the least two significant bit positions. Note that the rest of the
second byte is zero-filled.
Table 7–1: GE MULTILIN MODBUS FUNCTION CODES
FUNCTION CODE DEFINITION DESCRIPTION SUBSTITUTE
HEX DEC
01 1 READ BINARY STATUS Read one or more consecutive binary status
bits from the 750/760.
---
02 2 ---
03 3 READ ACTUAL VALUES OR
SETPOINTS
Read actual value or setpoint registers from one
or more consecutive memory map register
addresses.
04H
04 4 03H
05 5 EXECUTE OPERATION Perform 750/760 specific operations. 10H
06 6 STORE SINGLE SETPOINT Write a specific value into a single setpoint
register.
10H
10 16 STORE MULTIPLE SETPOINTS Write specific values into one or more
consecutive setpoint registers.
---
MASTER QUERY MESSAGE: EXAMPLE (HEX):
SLAVE ADDRESS 11 query message for slave 11
FUNCTION CODE 01 read binary status
DATA STARTING ADDRESS: high order byte, low order byte 00 13 data starting at address 0013h
NUMBER OF DATA BITS: high order byte, low order byte 00 0A 10 data bits
CRC: low order byte, high order byte 4F 58 computed cyclic redundancy check
SLAVE RESPONSE: EXAMPLE (HEX):
SLAVE ADDRESS 11 response message from slave 11
FUNCTION CODE 01 read binary status
BYTE COUNT 02 2 bytes total
DATA BYTES: high order byte, low order byte 4D 02 register value in address 0013h = 4D02h
CRC: low order byte, high order byte CC AE computed cyclic redundancy check code
Courtesy of NationalSwitchgear.com
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