Contemporary Control Systems BASremote User Manual (firmware 3.1.x) - Manual (Page 18)

TD040300-0MF
18
4.2 Connecting Expansion Equipment
Input/output points beyond those available from the BAS Remote Master module can
be increased by adding BAS Remote Expansion modules or by attaching Modbus serial
devices to the MB bus. The MB port is used for connecting to 2-wire Modbus serial
devices while the DN port is used for connecting to BAS Remote Expansion modules.
Both ports are non-isolated EIA-485 compatible.
When installing equipment, make a record that identifies the power source, equipment
locations, IP and MAC ID numbers, protocol in use, baud rate, cable colour coding, etc.
— anything that will be helpful for future staff.
4.2.1 BAS Remote Expansion Module Connections
Expansion modules are intended to occupy positions to the right or left of the Master
module on the same DIN-rail or on additional DIN-rails within the same control panel. In
this situation only a short 2-wire twisted-pair cable is needed for making connections
between DN on the Master module and UP on the first Expansion module. Up to three
Expansion modules can attach to the Master module using a daisy-chain wiring
scheme. The second Expansion module has its UP port connected to the preceding
Expansion
module’s DN port. The last Expansion module will have a vacant UP port.
The D+ terminal on one device must attach to the D+ terminal on the other. The same
applies to the D
– terminals. Bias and termination exists on the UP terminals. See
Figure 6 for wiring details. For short connections, unshielded cable can be used.
Expansion modules are automatically assigned Modbus addresses beginning with 2
based upon its position to the Master within the daisy-chain wiring.
4.2.1.1 Bias and Termination
Each Master and Expansion module has two Expansion ports. The Master ports are
labelled MB and DN; the Expansion ports are UP and DN. These are shared buses
where only one device drives the bus at any one time. When no device is driving the
bus, the bus floats. To prevent noise from being interpreted as data, the bus must be
biased to a valid state. (The Modbus Serial specification calls this polarization.) With
no data on the bus, the D+ pin is biased to be more positive than the D
– pin. Bias is
applied at only one point on the bus: the Master provides bias internally on its MB port,
the Expansion does so on its UP port.
4.2.1.2 Communicating from Master to Expansion Modules
The Master uses the downstream port DN to communicate to the upstream port UP on
the Expansion. If additional Expansion modules are used, they are cascaded such that
the DN port of the Expansion module nearest the Master is connected to the UP port on
the added Expansion module. Commands received by an Expansion
module’s UP port
are relayed to its DN port while being read by the module itself. Similarly, a response
received at the DN port is transferred to the UP port
— eventually arriving at the Master.
Thus all connections (Master-Expansion and Expansion-Expansion) are point-to-point
with termination and bias in each UP transceiver. DN ports have termination only.