7-4 239 Motor Protection Relay GE Multilin
7 COMMUNICATIONS 7 COMMUNICATIONS
7
7.6 TIMING
Data packet synchronization is maintained by timing constraints. The receiving device must measure
the time between the reception of characters. If three and one half character times elapse without a
new character or completion of the packet, then the communication link must be reset (i.e. 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.
7.7 239 SUPPORTED MODBUS FUNCTIONS
The following functions are supported by the 239:
• 03 - Read Setpoints and Actual Values
• 04 - Read Setpoints and Actual Values
• 05 - Execute Operation
• 06 - Store Single Setpoint
• 07 - Read Device Status
• 08 - Loopback Test
• 16 - Store Multiple Setpoints
7.8 03/04: READ SETPOINTS / ACTUAL VALUES
• Modbus implementation:Read Input and Holding Registers
• 239 Implementation: Read Setpoints and Actual Values
These commands can be used to read any Setpoint ("holding registers") or Actual Value ("input reg-
isters"). Holding and input registers are 16-bit (two byte) values transmitted low order byte first. Thus
all 239 Setpoints and Actual Values are sent as two bytes. The maximum number of registers that
can be read in one transmission is 125. Function codes 03 and 04 are configured to read setpoints or
actual values interchangeably because some PLCs do not support both of them.
The slave response to these function codes is the slave address, function code, a count of the data
bytes to follow, the data itself, and the CRC. Each data item is sent as a two byte number with the
low order byte sent first.
a) MESSAGE FORMAT AND EXAMPLE
Request slave 11 to respond with 3 registers starting at address 006B.
For this example the register data in these addresses is:
Address Data
006B 0000
006C 0000
006D 0000
Master Transmission Bytes Example (hex)
SLAVE ADDRESS - 1 byte 11 message for slave 11
FUNCTION CODE - 1 byte 03 read registers
DATA STARTING ADDRESS - 2 bytes 00 6B data starting at 006B
NUMBER OF SETPOINTS - 2 bytes 00 03 3 registers – 6 bytes total
CRC - 2 bytes 76 87 CRC calculated by the master
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