GE Multilin N60 Network Stability and Synchrophasor Measurement System B-3APPENDIX B B.2 MODBUS FUNCTION CODESBB.2MODBUS FUNCTION CODES B.2.1 SUPPORTED FUNCTION CODESModbus ® officially defines function codes from 1 to 127 though only a small subset is generally needed. The relay supportssome of these functions, as summarized in the following table. Subsequent sections describe each function code in detail.B.2.2 READ ACTUAL VALUES OR SETTINGS (FUNCTION CODE 03/04H)This function code allows the master to read one or more consecutive data registers (actual values or settings) from a relay.Data registers are always 16-bit (two-byte) values transmitted with high order byte first. The maximum number of registersthat can be read in a single packet is 125. See the Modbus memory map table for exact details on the data registers.Since some PLC implementations of Modbus only support one of function codes 03h and 04h. The N60 interpretationallows either function code to be used for reading one or more consecutive data registers. The data starting address willdetermine the type of data being read. Function codes 03h and 04h are therefore identical.The following table shows the format of the master and slave packets. The example shows a master device requestingthree register values starting at address 4050h from slave device 11h (17 decimal); the slave device responds with the val-ues 40, 300, and 0 from registers 4050h, 4051h, and 4052h, respectively.FUNCTION CODE MODBUS DEFINITION GE MULTILIN DEFINITIONHEX DEC03 3 Read holding registers Read actual values or settings04 4 Read holding registers Read actual values or settings05 5 Force single coil Execute operation06 6 Preset single register Store single setting10 16 Preset multiple registers Store multiple settingsTable B–3: MASTER AND SLAVE DEVICE PACKET TRANSMISSION EXAMPLEMASTER TRANSMISSION SLAVE RESPONSEPACKET FORMAT EXAMPLE (HEX) PACKET FORMAT EXAMPLE (HEX)SLAVE ADDRESS 11 SLAVE ADDRESS 11FUNCTION CODE 04 FUNCTION CODE 04DATA STARTING ADDRESS - high 40 BYTE COUNT 06DATA STARTING ADDRESS - low 50 DATA #1 - high 00NUMBER OF REGISTERS - high 00 DATA #1 - low 28NUMBER OF REGISTERS - low 03 DATA #2 - high 01CRC - low A7 DATA #2 - low 2CCRC - high 4A DATA #3 - high 00DATA #3 - low 00CRC - low 0DCRC - high 60
