Contemporary Control Systems BASremote User Manual (firmware 3.1.x) - Manual (Page 19)

TD040300-0MF
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4.2.2 Modbus Serial Bus Connections
The Modbus serial Expansion port (MB) on the BAS Remote Master module is non-
isolated EIA-485 compatible. When connecting to other non-isolated devices, care
must be exercised to ensure that all non-isolated Modbus devices share the same
ground reference (COM) with the BAS Remote Master module. This is usually
accomplished by sharing the same power source. Configure the Modbus baudrate and
protocol using the BAS Remote Modbus port web page.
Figure 5 — Internal Termination and Bias
When connecting to an isolated 3-wire Modbus device, the signal common of the
isolated device must be connected to the SC pin between the MB and DN ports. This ties
the two reference points together for reliable communications. Refer to Figure 6 for wiring
details.
Modbus serial device can only be attached to the MB port on the Master module. Refer
to Figure 5 for details on the bias and termination network present on the MB port.
Together,
these resistors approximate one 120 Ω terminating resistor. Terminal D+
represents the more positive connection for the EIA-485 Modbus serial network while
D
– represents the less positive connection. Make corresponding connections to
Modbus serial devices. The last device on the bus should have applied bias and
termination or just termination. A shielded twisted-pair cable should be used with
interconnecting devices. Connect the shields together and attached to chassis at only
one point. Refer to Figure 6 for wiring details.