Networking

Network Port Settings

The FAC-2100 supports RS-485 or RS-422 network communications. Selection of the physical network type, termination, and biasing options are set via dip-switch type jumpers internal to the unit.

Remove the side cover of the FAC-2100 (item 22 in the Parts List Exploded View) to gain access to the board with the dip-switches. The board with the dip switches is the Microprocessor and I/O Board (item 29 in the Parts List Exploded View). See the figure below for an image of the surface of the Microprocessor and I/O Board in the vicinity of the switches. Note that adjacent to the switch are labels for the switch function as well as an indicator for the direction of the ON and OFF positions.

Network Configuration Dip-Switches on Microprocessor and I/O Board

The switches have the following function:
SW485 Switch

Termination Resistor: In the ON position a termination resistor is added to the network wires. Note that if the network is in RS-422 mode that termination is added to the Rx+ and Rx- pair.
RS-485 Bias Resistors: These two switches should be set in the same position. If in the ON position, biasing resistors are added to the networking wires.

SW422 Switch

RS-422 ON/RS-485 OFF: In the ON position, the network port is configured for RS-422. In the OFF position, the network port is configured for RS-485.
RS-422 Bias Resistors: These two switches should be set in the same position. In the ON position, biasing resistors are added to the RS-422 2nd set of networking wires.

RS-485 Mode

The FAC-2100 RS-485 mode allows up to 32 RS-485 unit load devices to communicate with one another (the FAC-2100 is a 1/8th unit load device).

The RS-485 network topology is one of a straight line bus with each controller directly connected to the bus using as short a stub as possible. Each end of the network should be terminated with approximately 120 ohms and the network must be properly biased.

The network cable must be compatible with RS-485 and have a characteristic impedance of approximately 120 ohms. Note that Cat 5 Ethernet network cable is typically an acceptable alternate. RS-485 requires a cable consisting of two twisted pair signal wires (i.e. 2 wires in 1 pair) and one ground return. It is important to make sure that the twisted pair wires are connected to the RS-485 Tx/Rx+ and RS-485 Tx/Rx- signal pins.

RS-485 Dip Switch Settings

For RS-485 mode, keep all of SW422 switches in the OFF position (to the right). On two controllers in the network, activate the network biasing by setting their RS-485 bias resistors in the ON position (to the left). All the other controller in the network should have these switches in the OFF position (to the right). The controllers on each end of the bus should have their termination resistors set ON (to the left) or use external 120 ohm terminators.

Example RS-485 Network Cabling Topology

The figure above shows a typical RS-485 network. Note the following In this example:

The cable topology is that of a straight-line bus.
The controllers on the end of the bus have their termination switches activated.
Two controllers on the network (doesn't matter which two) have their bias switches activated.
Stubs (the cable length between the 'T' connection in the bus and the controller) are kept as short as possible. Ideally, there are no stubs and the wiring simply chains from controller to controller.

Example RS-485 Network Wiring

The above figure shows another example of RS-485 network wiring. In this case the network cabling goes from the COM 1 connecter of one controller to the COM 2 connector of the next controller in the bus. An alternative approach (which leaves the COM 1 connector open for connection to a configuration PC) is to go from COM 2 to COM 2 and splice the connections in the circular connector.

RS-422

Similar to RS-485, the network topology is one of a straight line bus with each controller directly connected to the bus using as short a stub as possible.

Again, similar to RS-485, the network cable must have a characteristic impedance of approximately 120 ohms. Cat 5 Ethernet network cable is typically an acceptable alternate. RS-422 requires a cable consisting of two sets of twisted pair signal wires (i.e. 4 wires in 2 pairs) and one ground return.

RS-422 differs from RS-485 in that the physical wiring will define one master station and a set of slave stations. The wiring is as follows:

The master station transmit lines (Tx+/Tx-) are attached to all the slave station receive lines (Rx+/Rx-) in parallel.
The master station receive lines (Rx+/Rx-) are attached to all the slave station transmit lines (Tx+/Tx-) in parallel.

In this setup the master station sends information to all the slaves. Slave stations (one at a time) can send information to only the master station. There is no mechanism for direct slave to slave communication – this virtual link must be made through software in the master.

Note that with the above configuration there are two channels and it is possible to have full duplex communication.

The following diagram shows the configuration:

Example RS-422 Network Connections

It is important to make sure that the twisted pair wires are connected to the RS-422 +/- pairs. In the diagram above, the tan and green lines each represent a twisted pair communication channel.

RS-422 Dip Switch Settings

Each twisted pair should be terminated with approximately 120 ohms at the most distant receiver and the pair must be properly biased.

Recall that in RS-422 mode the termination dip switch installs the resistor across the Rx+/Rx- lines.

Since the master's receiver is the only receiver on the green twisted pair, it should be have termination and biasing turned on. If possible, install the master at one end of the bus to ensure it is physically located at the end of the green channel.

The tan twisted pair represents the second channel on the bus. For the tan channel, activate the termination resistor on the slave at the remaining end of the bus (slave station n in the diagram below). At least one slave station must have its biasing resistors activated. For redundancy it is recommended to active two slave station RS-422 biasing (no more then two should be activated).

RS-422 Network Topology

For RS-422 mode, place the RS-422 ON/RS-485 OFF switch to the ON position (to the left).

For the controllers that need to install the receiver termination, set the termination Resistor to the ON position (to the left). Otherwise, set this switch OFF.

For the controllers that need to activate biasing, set the pair of RS-422 Bias Resistors to the ON position (to the left). Otherwise, these pair of switches should be to the right (OFF) position.

In all cases, the RS-485 Bias Resistors should both be set to the OFF position (to the right).

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