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How to How to 10/25/2022
Products: MP-V, RP-C
Functionalities: Hardware
Product version: 2022

Connecting an Isolated RS-485 Adapter

You connect an Isolated RS-485 Adapter (via a separate connection cable) to a BACnet/IP controller to convert the controller RS-485 RJ45 interface to screw terminals, which is required to connect the controller to a BACnet MS/TP (RS-485) network. You use an Isolated RS-485 Adapter (SXWISORS48510001) when the BACnet MS/TP network is comprised of isolated controllers, such as MNB BACnet devices (zone controllers) and selected others.

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The Isolated RS-485 Adapter is equipped with an RJ45 port for connection to a BACnet/IP controller RS-485 Com port and a 4-position removable screw terminal block for connection to a BACnet MS/TP (RS-485) network.

RJ45 port and screw terminals, Isolated RS-485 Adapter

RJ45 port and screw terminals, Isolated RS-485 Adapter

The following figure shows an example of how an Isolated RS-485 Adapter is connected to the RS-485 Com B port (RJ45) of an RP-C controller via a Cat 5 (or higher) UTP cable of maximum length 0.3 m (12 in.) and how the BACnet MS/TP network (RS-485) wires from the adapter are connected to the RS-485 Com port screw terminals of an AS-P server.

Example with an Isolated RS-485 Adapter used for connection of an RP-C controller to a BACnet MS/TP network and an AS-P server

Example with an Isolated RS-485 Adapter used for connection of an RP-C controller to a BACnet MS/TP network and an AS-P server

Connection of RJ45 port to BACnet/IP controller RS-485 Com port

A separate connection cable is required to connect the RJ45 port on the RS-485 adapter to the RS-485 Com port (RJ45) on the BACnet/IP controller. The cable is not included and needs to be purchased separately.

Use a Cat 5 (or higher) unshielded, straight-through wired cable with eight conductors (four twisted pairs) and RJ45 connectors. Use a cable with the wire size (cross-sectional area) 22 to 26 AWG (0.34 to 0.14 mm²), a maximum length of 0.3 m (12 inches), and a rating that meets the requirements of the target environment. For example, when devices are installed in a space that handles conditioned air or return air, the cables typically need to be plenum-rated.

Notice

LOSS OF COMMUNICATION

  • Ensure that the length of the cable for connection of the RS-485 adapter to the controller does not exceed 0.3 m (12 inches).

  • Use a Cat 5 or higher unshielded twisted pair cable with eight conductors (four twisted pairs), a cross-sectional area of 22 to 26 AWG (0.34 to 0.14 mm 2 ), and a rating that meets the requirements of the target environment.

Failure to follow these instructions can result in loss of communication.

For more information, see Connecting an Isolated RS-485 Adapter .

For more information, see RJ45 Pinout for the RS-485 Ports of the RP-C Controller .

Wiring of screw terminals for connection to BACnet MS/TP network

The four screw terminals are wired and connected to a BACnet MS/TP (RS-485) network as described in the following table.

Recommended screw tightening torque: 0.5 Nm (4.5 lbf.in)

Screw Terminals, Isolated RS-485 Adapter

Terminal

Usage

BIAS+

Provides +5 VDC for use to deliver a positive bias to the TX/RX+ data line through an appropriate value resistor. For example, when configuring strong dual end bias such as required by MNB BACnet devices, a 1 kohm resistor would be connected between the BIAS+ and TX/RX+ terminal.

TX/RX+

Data line (+) for connection to the TX/RX+ signal in the BACnet MS/TP twisted pair connecting all devices on the bus.

TX/RX–

Data line (–) for connection to the TX/RX– signal in the BACnet MS/TP twisted pair connecting all devices on the bus.

SHIELD

Provides common (low side) of 5 VDC supply for delivering negative bias to the TX/RX– data signal through an appropriate value resistor. For example, when configuring strong dual end bias such as required by MNB BACnet devices, a 1 kohm resistor would be connected between the SHIELD and TX/RX– terminal. The Isolated RS-485 Adapter removes the need for a separate 5 VDC supply when positioned at the end of the bus.

For more information, see Wiring the Screw Terminals on an Isolated RS-485 Adapter .

For more information, see RS-485 Communications .

Notice

BACNET MS/TP NETWORK OUTAGE OR PERFORMANCE DEGRADATION

For a BACnet MS/TP network with MNB BACnet devices, ensure you use an isolated RS-485 adapter (SXWISORS48510001) connected to the RP controller Com B port or MP-V controller Com A port.

Failure to follow these instructions can result in BACnet MS/TP network outage or performance degradation.

Notice

LOSS OF COMMUNICATION

  • Ensure that the length of the cable for connection of the RS-485 adapter to the controller does not exceed 0.3 m (12 inches).

  • Use a Cat 5 or higher unshielded twisted pair cable with eight conductors (four twisted pairs), a cross-sectional area of 22 to 26 AWG (0.34 to 0.14 mm 2 ), and a rating that meets the requirements of the target environment.

Failure to follow these instructions can result in loss of communication.

To connect an Isolated RS-485 Adapter
  1. Connect a Cat 5 (or higher) unshielded, straight-through wired cable with eight conductors (four twisted pairs) with RJ45 connectors to the RJ45 receptacle on the RS-485 adapter.

    Use a cable with the wire size (cross-sectional area) 22 to 26 AWG (0.34 to 0.14 mm²), a maximum length of 0.3 m (12 inches), and a rating that meets the requirements of the target environment, for example, plenum space.

    Note:

    The cable is not included.

  2. Connect the other end of the cable to the RP controller RS-485 Com B port or MP-V controller RS-485 Com A port.

    The RS-485 Com port needs to be configured for BACnet MS/TP (RS-485) network. For more information, see RP-C BACnet MS/TP Support .For more information, see Allocating Flexible Ports .

You can now install and wire the RS-485 adapter.

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