GE Power Quality Meter Series User Manual

7-2 PQM Power Quality Meter GE Power Management
7.1 OVERVIEW 7 MODBUS COMMUNICATIONS
7
7.1.4 DATA PACKET FORMAT
A complete request/response sequence consists of the following bytes (transmitted as separate data frames):
Master Request Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
Slave Response Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
•
SLAVE ADDRESS
: The first byte of every transmission. It represents the user-assigned address of the
slave device assigned to receive the message sent by the master. Each slave device must be assigned a
unique address so only it responds to a transmission that starts with its address. In a master request trans-
mission, the SLAVE ADDRESS represents the address to which the request is being sent. In a slave
response transmission the SLAVE ADDRESS represents the address sending the response.
A master transmission with a SLAVE ADDRESS of 0 indicates a broadcast command. Broadcast
commands can be used only to store setpoints or perform commands.
•
FUNCTION CODE
: This is the second byte of every transmission. Modbus defines function codes of 1 to
127. The PQM implements some of these functions. See section 3 for details of the supported function
codes. In a master request transmission the FUNCTION CODE tells the slave what action to perform. In a
slave response transmission if the FUNCTION CODE sent from the slave is the same as the FUNCTION
CODE sent from the master then the slave performed the function as requested. If the high order bit of the
FUNCTION CODE sent from the slave is a 1 (i.e. if the FUNCTION CODE is
>
127) then the slave did not
perform the function as requested and is sending an error or exception response.
•
DATA
: This will be a variable number of bytes depending on the FUNCTION CODE. This may be Actual
Values, Setpoints, or addresses sent by the master to the slave or by the slave to the master. See section
3 for a description of the supported functions and the data required for each.
•
CRC
: This is a two byte error checking code.
7.1.5 ERROR CHECKING
The RTU version of Modbus includes a 2-byte CRC-16 (16-bit cyclic redundancy check) with every transmis-
sion. The CRC-16 algorithm essentially treats the entire data stream (data bits only; start, stop and parity are
ignored) as one continuous binary number. This number is first shifted left 16 bits and then divided by a charac-
teristic polynomial (11000000000000101B). The 16-bit remainder is appended to the end of the transmission,
MSByte first. The resulting message including CRC, when divided by the same polynomial at the receiver,
results in a zero remainder if no transmission errors have occurred.
If a PQM Modbus slave device receives a transmission in which an error is indicated by the CRC-16 calcula-
tion, the slave device will not respond to the transmission. A CRC-16 error indicates that one or more bytes of
the transmission were received incorrectly and thus the entire transmission should be ignored in order to avoid
the PQM 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.
NOTE

GE Power Quality Meter Series Specifications

General

Brand	GE
Model	Power Quality Meter Series
Category	Measuring Instruments
Language	English