Aerco Bms Ii/Acs Master To Bcm Slave Wiring Connections; Ems Or Bas Master To Bcm Slave Wiring Connections; Rs485 Loop Termination Resistors And Bias - Watts AERCO EXT Installation, Operation & Maintenance Manual

Ufly Controller and BCM for Modulex EXT
CHAPTER 6: BOILER COMMUNICATIONS MODULE (BCM)
Figure 6-12: BCM Modbus (RS485) Connections

6.5.3 AERCO BMS II/ACS Master to BCM Slave Wiring Connections.

The AERCO BMS II/ACS contains a RS232 port to connect to an EMS/BAS or personal computer,
plus a RS485 port for connection to the BCM's Modbus input. See section 6.7 for details.

6.5.4 EMS or BAS Master to BCM Slave Wiring Connections

When a third-party EMS or BAS Master is used, the Modbus Network connections will depend on
the available communication port(s) on the EMS/BAS. Many EMS/BAS Models contain only a
RS232 (DB9) port, while others contain either a 2-Wire or 4-Wire RS485 port. In addition, some
EMS/BAS models contain both a RS232 and a RS485 port. If the EMS or BAS is equipped with
only a RS232 port, a RS232-to-RS485 converter will be required (such as a B&B Electronics,
Model 485SD9TD or AERCO Part No. 124943). See section 6.7 for details.

6.6 Rs485 Loop Termination Resistors And Bias

A terminating resistor (120 ohms) on each end of the RS485 loop is designed to match the
electrical impedance characteristic of the twisted-pair loop and prevent echoes or cross-talk from
corrupting data on the line. Short or medium length Modbus/RS485 loops (less than 1000 feet)
can usually operate satisfactorily without the terminating resistor. However, longer loop runs (over
1000 feet), may require terminating resistors.
Bias may be necessary on the RS485 loop to minimize noise on the circuit. AERCO recommends
that both terminating resistors and bias be implemented on the RS485 circuit as described below.
OMM-0159_A_A • 6/28/2022 Technical Support • (800) 526-0288 • Mon-Fri, 8 am - 5 pm EST
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