GE 489 Communications Manual

Also see for Multilin 489: Instruction manual Instruction manual Instruction manual Instruction manual

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GE 489 Communications Manual Manual pdf 9 page image

489 GENERATOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE 5CG.2 Modbus FunctionsCG.2.1 Supported FunctionsThe following functions are supported by the 489:• Function Codes 03 and 04: Read Setpoints and Actual Values• Function Code 05: Execute Operation• Function Code 06: Store Single Setpoint• Function Code 07: Read Device Status• Function Code 08: Loopback Test• Function Code 16: Store Multiple SetpointsA detailed explanation of how the 489 implements these function codes is shown in thefollowing sections.CG.2.2 Function Codes 03/04: Read Setpoints / Actual ValuesModbus implementation: Read Input and Holding Registers489 Implementation: Read Setpoints and Actual ValuesFor the 489 Modbus implementation, these commands are used to read any setpoint(‘holding registers’) or actual value (‘input registers’). Holding and input registers are 16-bit(two byte) values transmitted high order byte first. Thus all 489 setpoints and actual valuesare sent as two bytes. The maximum of 125 registers can be read in one transmission.Function codes 03 and 04 are configured to read setpoints or actual valuesinterchangeably since some PLCs do not support both function codes.The slave response to these function codes is the slave address, function code, a count ofthe number of data bytes to follow, the data itself and the CRC. Each data item is sent as atwo byte number with the high order byte sent first. The CRC is sent as a two byte numberwith the low order byte sent first.Message Format and Example:Request slave 11 to respond with 2 registers starting at address 0235. For this example, theregister data in these addresses is:Address Data0235 00640236 000AMaster Transmission Bytes Example DescriptionSlave Address 1 0B message for slave 11Function Code 1 03 read register valuesData Starting Address 2 02 32 data starting at 0235hNumber of Setpoints 2 00 02 2 registers = 4 bytes total