Serial Port Control - Cisco IR829 Hardware Installation Manual

RS485 Termination Resistors
• Half Duplex - one pair shall be used for TX/RX+ and TX/RX- signals (orange and white/orange) and
• Full Duplex - one pair shall be used for RX+ and RX- signals (green and white/green), one pair shall be
• Unused wires should be terminated with 100 ohm resistors to ground at both ends of the cable using your
RS485 Termination Resistors
The following are some best practices when using terminating resistors:
• When the bit time is not substantially longer than the loop time of the cable, termination is crucial for
• If the bit time is substantially longer than the loop time of the cable, you can usually get by without
• For standard termination, you would match the termination resistor value with the differential-mode
• No 120 Ohm terminations are used on the other multi-drop ports in the middle of the network, only at
RS485 Failsafe Resistors
The following are some best practices when using failsafe resistors:
• When inputs are between -200mV and +200mV, receiver output is "undefined". There are four common
• Fail-safe biasing is used to keep the receiver's output in a defined state when one of these conditions
• The fail-safe biasing consists of a pull-up resistor on the non-inverting line and a pull-down resistor on
• With proper biasing, the receiver will output a valid high when any one of the fault conditions occurs.
• Fail-safe bias resistors should be placed at the receiver end of the transmission line.

Serial Port Control

The entire layer 1 configuration of the IR809 / IR829 serial ports is done in IOS under the following constructs.
The following example shows values other than the defaults to show most of the IOS keywords.
interface Async0 (or Async 1)
no ip address
encapsulation relay-line
line 1 (or line 2)
transport preferred none
transport input all
transport output all
databits 7
parity odd
Cisco IR829 Industrial Integrated Services Router Hardware Installation Guide
40
one wire for GND (blue).
used for TX+ and TX- signals (orange and white/orange), and one wire for GND (blue).
chosen wire termination method, e.g. screw terminals, punch down blocks, etc. This will help to avoid
undesirable inductive pickup in the unused conductors.
minimizing reflections. (Speed for the signal to go the length of the cable and reflect back)
terminating resistors as can be observed with short point to point cables at lower baud rates.
characteristic impedance of the cabling on both ends of the network. Typically 120 Ohm.
the two far ends of the network.
fault conditions that result in the undefined receiver output that can cause erroneous data:
• All transmitters in a system are in shutdown.
• The receiver is not connected to the cable.
• The cable has an open.
• The cable has a short.
occurs.
the inverting line.
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