Modulex Controller Operation
6
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.
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.
accomplished
Loop bias is
by
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.
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).
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.
The last unit in the chain must be energized (even if disabled)
to enable bias.
Figure 1-9. BCM Loop Termination and Bias
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
as a guide and does not include
to the EMS/BAS manufacturer’s
any network
diagram
BCM Controller Slaves. This
with a RS485
Master EMS/BAS
converter will
r. It should be noted that this diagram is only intended
all possible scenarios. Refer
manual prior to attempting
wiring connections.
1.4.4 EMS OR BAS MASTER TO BCM
SLAVE WIRING CONNECTIONS
1.4.5 RS485 LOOP TERMINATION
RESISTORS AND BIAS
1.5
EMS/BAS TERMINATING RESISTOR
AND BIAS
1.5.1 BCM CONTROLLER TERMINATING
RESISTOR AND BIAS
1.6
MODBUS NETWORK WIRING
DIAGRAM
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.
BCM FRONT VIEW
