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Modulex Controller Operation

1.4.4 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.

1.4.5 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.

activating pull-up/pull-down resistors on the last Boiler

Communications Module (BCM) in the chain.

Modulex recommends that both terminating resistors

and bias be implemented on the RS485 circuit as

described in paragraphs 1.5 and 1.5.1 which follow.

1.5

EMS/BAS TERMINATING RESISTOR

AND BIAS

When an 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 1-9).

1.5.1 BCM CONTROLLER TERMINATING

RESISTOR AND BIAS

BCMs 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 BCM Controller in the chain. Enabling the

terminating resistor is accomplished by positioning jumper

JP1 as shown in Figure 1-9 only on the BCM Board

contained in the last BCM Controller.

As mentioned in paragraph 1.5.1, when the controlling

Master is an EMS or BAS, pull-down bias may be

implemented by connecting a 1K ohm resistor as shown

in Figure 1-9.

Loop bias is

accomplished

The last unit in the chain must be energized (even if disabled)

to enable bias.

BCM FRONT VIEW

Figure 1-9. BCM Loop Termination and Bias

1.6 MODBUS NETWORK WIRING

DIAGRAM

Figure 1-10 shows a Sample Modbus Network wiring

for a Master EMS/BAS controlling

Figure shows an EMS or BAS Master equipped

port. If the EMS or BAS contains a 4-Wire RS485 port, refer to

Detail "A" for wiring details. If the controlling

contains only a RS232 port, a RS232-to-RS485

be required to interface with connector Y2 at each BCM

Controlle

r. It should be noted that this diagram is only intended

as a guide and does not include

to the EMS/BAS manufacturer's

any network

wiring connections.

CAUTION

It is imperative that polarity be maintained

between all Modbus Network connections. The

Network will not operate if the proper polarity is

not maintained. Also, twisted-pair wiring shields

should only be terminated at the controlling

Master for the Modbus Network.

diagram

BCM Controller Slaves. This

with a RS485

Master EMS/BAS

converter will

all possible scenarios. Refer

manual prior to attempting