Communications; Modbus Protocol; Electrical Interface; Data Frame Format / Data Rate - GE 239 Instruction Manual

7 COMMUNICATIONS

7 COMMUNICATIONS
239 INSTRUCTION MANUAL 7 COMMUNICATIONS

7.1 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 inter-
face card allowing direct connection of relays. Although the Modbus protocol is hardware indepen-
dent, 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 config-
uration up to 32 slaves can be daisy-chained together on a single communication channel.
The GE Multilin 239 is always a Modbus slave. It cannot be programmed as a Modbus master. Com-
puters or PLCs are commonly programmed as masters. The Modbus protocol exists in two versions:
Remote Terminal Unit (RTU, binary) and ASCII. Only the RTU version is supported by the 239. Mon-
itoring, programming and control functions are possible using read and write register commands.

7.2 ELECTRICAL INTERFACE

The hardware or electrical interface is two-wire RS485. In a two-wire RS485 link data flow is bi-direc-
tional 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 net-
work 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 2.3: EXTERNAL CONNECTIONS on page 2–3 for details on correct serial port
wiring.

7.3 DATA FRAME FORMAT / DATA RATE

One data frame of an asynchronous transmission to or from a 239 consists of 1 start bit, 8 data bits,
and 1 stop bit. This produces a 10 bit data frame. This is important for transmission through modems
7
at high bit rates (11 bit data frames are not supported by Hayes modems at bit rates of greater than
300 bps).
Modbus protocol can be implemented at any standard communication speed. The 239 supports
operation at 1200, 2400, 4800, 9600, and 19200 baud.
GE Multilin 239 Motor Protection Relay 7-1
Courtesy of NationalSwitchgear.com