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FLEXNET – FNER Expandable (Modular) Routing Switches – Installation & Operation Manual
CHAPTER 6: RS-485
6.2.5 Master/Slave
Most RS-485 systems use Master/Slave architecture, where each slave unit has its unique address and responds only to
packets addressed to this unit. These packets are generated by the master, which periodically polls all connected slave
units. Only communications sent to the assigned address will cause a response by the MPC 128 on the bus. This is why it is
important to assign the MPC 128 on your 485 bus a unique address.
6.2.6 4-Wire & 2-Wire Systems
RS-485 has a 2-wire and a 4-wire standard. FNER uses the 4-wire standard. This allows master to send data out one pair of
wires and the slaves sends data back to the master on the other twisted pair.
6.2.7 Cable Lengths
FLEXNET recommends the same cable length as the RS-485 standard. Your RS-485 cable length can be up to 4000 feet
between nodes.
6.2.8
Signal Reflection
As data speeds and cable lengths go up, the problem of signal reflections becomes important. Lines must be properly
terminated by a resistor that makes the cable look electrically like it is infinitely long. An infinitely long cable, of course, can
have no reflected signals because the far end is infinitely far away. When this reflection occurs it propagates from one end of
the wires to the other end and back repeatedly. Therefore, to add something to fix this reflection you only need to take care
of the end of the cable, not any nodes in between the ends. When devices are daisy chained together, the MPC 128 between
the first and last unit in the chain do not need to have termination resistors. If the MPC 128 is the last in line of a RS-485 bus
then the control termination should be on.
6.2.9 Receiver/Transmitter Termination
RS-485 termination is required to eliminate signal reflection as discussed earlier. There is a twisted pair for sending data
from the PC to the MPC 128 and there is another twisted pair for the MPC 128 to transmit to the PC. Therefore, in a four
wire RS-485 bus you only need to terminate the receivers. FNER RS-485 termination therefore is only applied to the internal
receivers and not the transmitters.
6.2.10 Biasing Resistors
When an RS-485 network is an idle state, all nodes are in the receive mode and the transmitters are tri-stated. When this
happens the state of the line is undefined. Bias resistors are nothing more than a pull up resistor on line B and pull down
resistor on line A to guarantee logic high when the bus is idle. When RS-485 termination is turned on these two resistors and
the termination resistor are connected to the RS-485 receiver.
6.3 Which Ports are RS-485?
The Control In port on the MPC 128 is the only RS-485 port available on an FNER system. Also to use RS-485 communications
the port has to be set up in 485 mode.
6.4 Summary
So now, we understand from the previous sections that FNER RS-485 communications only needs to terminate the ends
of the cables and we only need to terminate the receivers. Internally the MPC 128 adds a termination resistor and two bias
resistors when termination is turned on.
6.5 Front Panel RS-485 Termination
Option 3 of the Serial Interface Menu opens the RS-485 Termination screen shown in Figure 8.
To get to the RS-485 Termination screen from the main menu: key sequence 2, 2, 1, 3.
2. System Options
2. Interface Options
1.
Serial
3.
Terminate
RS-485
Figure 20: RS-485 Termination
