GE 239 Motor Protection Relay

GE 239 Motor Protection Relay

Modbus Protocol

The GE Multilin 239 implements a subset of the AEG Modicon Modbus RTU serial  communication standard. Many popular programmable controllers support this  protocol directly with a suitable interface card allowing direct connection of relays.  Although the Modbus protocol is hardware independent, the 239 interface uses a 2  wire RS485 hardware interface. Modbus is a single master multiple slave protocol  suitable for a multi-drop configuration as provided by RS485 hardware. In this  configuration up to 32 slaves can be daisy-chained together on a single  communication channel.

Electrical Interface

The hardware or electrical interface is two-wire RS485. In a two-wire RS485 link data  flow is bi-directional and half duplex. That is, data is never transmitted and received at  the same time. RS485 lines should be connected in a daisy chain configuration (avoid  star connections) with a terminating network installed at each end of the link, i.e. at  the master end and at the slave farthest from the master. The terminating network  should consist of a 120 Ω resistor in series with a 1 nF ceramic capacitor when used  with Belden 9841 RS485 wire. The value of the terminating resistors should be equal to  the characteristic impedance of the line. This is approximately 120 Ω for standard #22  AWG twisted pair wire. Shielded wire should always be used to minimize noise. Polarity is important in RS485 communications. Each '+' terminal of every device must be  connected together for the system to operate. See Section 2.1.3 in the 239 Instruction  Manual for details on correct serial port wiring.

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

03/04: Read Setpoints / Actual Values 

• Modbus implementation: Read Input and Holding Registers 

• 239 Implementation: Read Setpoints and Actual Values

Execute Operation 

• Modbus Implementation: Force Single Coil 

• 239 Implementation: Execute Operation

Store Single Setpoint 

• Modbus Implementation: Preset Single Register 

• 239 Implementation: Store Single Setpoint

Read Device Status 

• Modbus Implementation: Read Exception Status 

• 239 Implementation: Read Device Status

Performing Commands 

Some PLCs may not support execution of commands using function code 5 but do  support storing multiple setpoints using function code 16. To perform this operation  using function code 16 (10H), a certain sequence of commands must be written at the  same time to the 239. The sequence consists of: Command Function register,  Command operation register and Command Data (if required). The Command  Function register must be written with the value of 5 indicating an execute operation  is requested. The Command Operation register must then be written with a valid  command operation number from the list of commands shown in the memory map.  The Command Data registers must be written with valid data if the command  operation requires data. The selected command will execute immediately upon  receipt of a valid transmission.