Specifications

MODBUS COMMUNICATION MANUAL
4-7
4.2.3 C-More Slaves To BMS or EMS Master
Wiring connections between a BMS Master and up to 32 Network C-More Boiler Controller
Slaves are made directly between the BMS internal RS485 terminals (Figure 4-4) and the
RS485 COMM terminals in each Boiler’s I/O Box (Figure 4-5).
If a third-party EMS Master is used in place of the BMS, the Modbus Network connections will
depend on the available communication port(s) on the EMS. Many EMS Models contain only a
RS232 (DB9) port, while others contain either a 2-Wire or 4-Wire RS485 port. Some EMS
models contain both a RS232 and a RS485 port. If the EMS is equipped with only a RS232
port, a RS232-to-RS485 converter will be required (such as a B&B Electronics, Model
485SD9TD).
4.3 RS485 LOOP TERMINATING 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), 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 C-More Boiler Controller in the
chain.
AERCO recommends that both terminating resistors and bias be implemented on the RS485
circuit as described in paragraphs 4.3.1 and 4.3.2 which follow.
4.3.1 BMS Terminating Resistor
Each BMS is equipped with a built-in terminating resistor (120 ohms) on the RS485 port.
Therefore, only one additional terminating resistor will be required at the other end of the RS485
loop. Ensure that the last C-More Boiler Controller Slave on the loop has its terminating resistor
activated as described in paragraph 4.3.2.
4.3.2 C-More Boiler Controller Terminating Resistor and Bias
C-More Boiler Controllers can function only as Slave devices on a Modbus Network. Since the
Slaves are connected in a “Daisy-Chain” configuration, the terminating resistor must be enabled
only in the last
C-More Boiler Controller in the chain. In addition, bias must also be implemented
only in the last C-More Boiler Controller. This is accomplished by setting a DIP switches on the
Primary Micro-Controller (PMC) Board contained in the applicable C-More Boiler Controller. The
last unit in the chain must be energized (even if disabled) to enable bias.
To activate the DIP switches, proceed as follows:
1. Remove power from the last C-More Boiler Controller in the RS485 loop.
2. Loosen and remove the four (4) screws securing the front panel assembly to the chassis as
shown in Figure 4-6.
3. Carefully separate the panel from the chassis. Use care to avoid applying undue stress to
the ribbon cable connected between the back of the panel and the chassis-mounted printed
circuit boards.