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750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
FUNCTION CODE 01H/02H: READ BINARY STATUS
CHAPTER 3: MODBUS FUNCTIONS
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
DNP 3.0 Device Profile Document
on page 5-1 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 position. 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.
Function Code 03h/04h: Read Actual Values /
Setpoints
This function code allows the master to read one or more consecutive data registers
(actual values or setpoints) from an 750/760. Data registers are always 16 bit (two byte)
values transmitted high-order byte first. The maximum number of registers that can be
read in a single packet is 120. See the section
Modbus Memory Map
on page 4-1 for exact
details on the data registers.
Master Transmission
Bytes
Example
Description
Slave Address
1
11
message for slave 11
Function Code
1
01
read binary status
Data Starting Address
2
00 13
data starting at 0013h
Number of Data Bits
2
00 0A
10 data bits
CRC
2
4F 58
computed CRC error code
Slave Response
Bytes
Example
Description
Slave Address
1
11
message from slave 11
Function Code
1
01
read binary status
Byte Count
1
02
2 bytes total
Data Bytes (high, low)
2
4D 02
value in address 0013h
CRC (low, high)
2
CC AE
computed CRC error code