B&B Electronics MESR424T_D-MT_D-SC - Manual - Manual (Page 16)

3. Setup and Connections
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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 MESR424
Depending upon the model, MESR424 Modbus Gateway can be configured to support
RS-232, RS-422 and RS-485 (2- and 4-wire) serial connectors. The unit may have DB-
9M connectors or 5-position terminal blocks.
If you select RS-232 mode when you configure the MESR424 Modbus Gateway, you
must connect the Modbus serial network to the MESR424 Modbus Gateway via a serial
cable. The MESR424 Modbus Gateway 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 DCT 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)
Figure 15. MESR RS-232 Connections
If you select RS-422 mode, RS-485 2-wire mode, or RS-485 4-wire mode when you
configure the MESR424 Modbus Gateway, you must connect the Modbus network
appropriately, via the 5-position terminal block or the DB9 connector.