Contemporary Control Systems BASrouter Installation Guide (Firmware 2.x) - Manual (Page 11)

TD071200-0IK
11
5 Operation
5.1 MS/TP Port
The MS/TP port uses an isolated EIA-485 transceiver connected to
a 3-pin removable connector (Figure 1). The transceiver + and –
lines tie to + and – pins of the connector. The SC pin ties to the
transceiver ground. You MUST connect the SC pin to the MS/TP
segment signal common — or MS/TP communication will fail!
In Figure 11 a simplified schematic of the isolated EIA-485
transceiver is shown. Notice that the transceiver common (SC)
is electrically isolated from the router power supply common
(COM) through the use of opto-isolators and an isolated DC-DC
converter. An isolated EIA-485 transceiver offers less chance of
damage to the transceiver from high common-mode voltages, but
there is still a risk. Surge suppression is applied between the two
differential inputs and between each input and ground. Since the
transceiver is electrically isolated, the transceiver circuit common
must be brought out for connection to the other devices on the
MS/TP network. This is accomplished by pin SC. The differential
pair is labelled + and – and corresponds to the same designations in
the BACnet MS/TP standard. This requires a three-wire connection
to the MS/TP network. Connecting other isolated MS/TP devices
is no problem since each device requires a 3-wire connection.
Just make corresponding connections. For non-isolated MS/TP
devices, usually only a 2-wire connection is provided. The third
wire is the power supply common. In this situation, the SC
connection MUST be made to the power supply common of the
non-isolated device. Refer to the device vendor instructions for
connecting such a device.
In a router application where one connection is made to Ethernet
and the other to MS/TP, the location of the router is probably at
the end of the MS/TP bus segment and therefore both bias and
termination must be applied to the segment end. The router is
shipped with bias and termination applied, but this can be changed
by removing three jumpers. This should be done if the router is
to be connected anywhere between the end MS/TP devices.