AERCO Control System (ACS) - Manual (Page 22)

MC2: 04/23/13 Page 22 of 144
GF-131
AERCO Control System (ACS)
AERCO International, Inc. • 100 Oritani Dr. • Blauvelt, New York 10913 • Phone: 800-526-0288
Installation, Operation, and Maintenance Manual
OMM-0081_0D
2.6 RS485 (MODBUS) WIRING AT THE ACS
The ACS communicates with the AERCO Boilers over a RS485 network using Modbus protocol.
All Modbus networks are implemented utilizing a Master/Slave scenario where only one device,
the Master, can initiate a communication sequence. AERCO Boilers equipped with C-More or
E8/BCM (Modulex) control systems can only function as Slaves on a Modbus network.
However, the ACS can function as a Master controlling C-More or BCM Boiler Slaves, or as a
Slave controlled by an Energy Management System (EMS) developed by other manufacturers.
Additional information on implementing Modbus networks is provided in AERCO Modbus
Communication Manual GF-114.
RS485 (Modbus) networks are wired in a “daisy chain” configuration similar to the example
shown in Figure 2-5. Shielded twisted-pair, 18 – 24 AWG cable (Belden #9841, #3105A, #8760
or equiv.) is recommended for RS485 wiring connections.
MASTER
SLAVE
#1
SLAVE
#2
SLAVE
#3
SLAVE
#4
SLAVE
#5
Figure 2-5: Typical Daisy-Chain Modbus/RS485 Network
At the ACS, RS485 (Modbus) wiring connections are made at the wiring terminals labeled 485
B+ and 485 A-. The cable shield is terminated at the SHLD terminal (3) of the ACS.
2.6.1 ACS Bias Switches & Loop Termination Resistors
Each ACS contains a built-in 120 ohm loop termination resistor and two bias DIP switches
which are mounted on the lower portion of the motherboard ( Figure 2-6). The 120 ohm
termination resistor is always active and is designed to match the electrical impedance at the
ACS end of the RS485 loop. Therefore, only one additional termination resistor will be required
at the boiler end of the RS485 loop. The two bias switches are used to activate or deactivate
bias voltage on the RS485 network and should be positioned as follows, depending on the type
of AERCO boilers being used. See the instructions below:
NOTE
Refer to paragraph 2.7 for detailed instructions and illustrations describing
how to implement loop termination, bias and wiring connections on the
RS485 loop.
Positioning the Two Bias Switches
1. When wiring to Boiler Control Modules (BCMs) controlling Modulex Boilers, the ACS bias
switches must be activated by placing them in the on (down or towards wall) position.
Loop termination is accomplished by activating the termination resistor in the last BCM
on the RS485 loop.
2. When wiring to C-More control systems (Benchmark & KC1000 Boilers), the positions of
the ACS bias switches will depend on which one of the two available methods is used.
Loop termination will also depend on the method employed. See paragraph 2.7 for
details.