B&B Electronics MESR321_SL_ML - Manual - Manual (Page 14)

Setup and Connections Vlinx MESR321 Modbus Gateway
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MESR321-xx_R001_2113m
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is positive relative to the A line in the idle (mark) state. Ground provides a
common mode reference.
RS-485 connections support 2-wire or 4-wire operation.
When configured for 4-wire operation the connection supports two signal pairs:
RXA(-), RXB(+) and TXA(-), TXB(+), plus GND. This makes full-duplex operation
possible. The data lines are differential pairs (A & B) in which the B line is
positive relative to the A line in the idle (mark) state. Ground provides a common
mode reference.
When configured for 2-wire operation the connection supports one signal pair:
DataB(+) and DataA(-) signal channels using half-duplex operation. The data
lines are differential with the Data B line positive relative to Data A in the idle
(mark) state. Ground provides a common mode reference.
Connecting the MESR321
The MESR321 has one DB9M serial connector that supports RS-232; it also has
a 5-position terminal block connector that supports RS-422 and RS-485 (2- and
4-wire).
If you select RS-232 mode when you configure the Modbus gateway, you must
connect the Modbus serial network to the Modbus gateway via a serial cable.
The MESR321 is a DTE. If the Modbus network is a DTE, use a null modem
(cross-over) cable. If the Modbus network is a DCE, use a straight-through cable.
DTE and DCE ports are complementary, the Output signals on a DTE port are
Inputs to a DCE port, and Output signals on a DCE port are Inputs to a DTE
port. The signal names match each other and connect pin for pin. Signal flow is
in the direction of the arrows. (See figure below)