06/05/13
AERCO International, Inc.
•
100 Oritani Dr.
•
Blauvelt, NY 10913
•
Ph: 800-526- 0288
Page
69
of
112
GF-115-C
Modulex E8 Controller and BCM
Operations and Maintenance Manual
OMM-0084_0D
Figure 7-8: Location of 2-Position and 3-Position Switches
7.4.4 AERCO BMS II/ACS Master to BCM Slave Wiring Connections.
The AERCO BMS II/ACS contains a RS232 port for connection to an EMS/BAS or personal
computer. In addition, the BMS II/ACS contains a RS485 port for connection to the BCM’s
Modbus input.
7.4.5 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).
7.5 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. Loop bias is
accomplished by activating pull-up/pull-down resistors on the last Boiler Communications
Module (BCM) in the chain. AERCO recommends that both terminating resistors and bias be
implemented on the RS485 circuit as described in
paragraphs 7.5.1
and
7.5.2
which follow.
7.5.1 Master AERCO BMS II/ACS or EMS/BAS Terminating Resistor and Bias
All AERCO BMS II/ACS units are equipped with a built-in terminating resistor. Therefore, when
the BMS II or ACS is the controlling Master, no terminating resistor needs to be added. In
addition, the BMS II/ACS contains two Bias DIP switches which must be activated when the
BMS II/ACS is the controlling Master. Refer to BMS II Manual
GF-124/ACS Manual GF-131
,
paragraph 2.6
and
Figure 2-6
for additional information on these switches.
When a third-party EMS or BAS is used as the controlling Master, consult the manufacturer’s
Technical Manual for termination resistor recommendations. If the EMS/BAS being used does
not provide a bias and one is needed, pull-down bias can be implemented by installing a 1K
ohm resistor (not provided) across terminals 2 and 4 on BCM connector Y2 (
Figure 7-9
).
