Data Format & Data Rate; Data Packet Format - GE Multilin PQM Instruction Manual

OVERVIEW
7.1.3 Data Format &
Data Rate
7.1.4 Data Packet
Format
7–2
Courtesy of NationalSwitchgear.com
One data frame of an asynchronous transmission to or from a PQM consists of 1 start bit, 8
data bits, and 1 stop bit, resulting in a 10-bit data frame. This is important for high-speed
modem transmission, since 11-bit data frames are not supported by Hayes modems at bit
rates greater than 300 bps. The Modbus protocol can be implemented at any standard
communication speed. The PQM supports operation at 1200, 2400, 4800, 9600, and 19200
baud.
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 transmission, 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.
NOTE
• 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.
CHAPTER 7: MODBUS COMMUNICATIONS
PQM POWER QUALITY METER – INSTRUCTION MANUAL