Timing; Supported Modbus Functions; 03/04: Read Setpoints / Actual Values - GE 239 Instruction Manual

7 COMMUNICATIONS
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.
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
• Modbus implementation:Read Input and Holding Registers
• 239 Implementation:
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
7
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
006B
006C
006D
Master Transmission
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
7-4
Courtesy of NationalSwitchgear.com
Read Setpoints and Actual Values
Data
0000
0000
0000
Bytes
- 1 byte
- 1 byte
- 2 bytes
- 2 bytes
- 2 bytes
239 Motor Protection Relay

7.7 239 SUPPORTED MODBUS FUNCTIONS

7.8 03/04: READ SETPOINTS / ACTUAL VALUES

Example (hex)
11 message for slave 11
03 read registers
00 6B data starting at 006B
00 03 3 registers – 6 bytes total
76 87 CRC calculated by the master
7 COMMUNICATIONS

7.6 TIMING

GE Multilin