Network Design Considerations 2-15
SIMATIC TIWAY I Series 500 NIM User Manual
2.4 Local Line Biasing
The TIWAY I Local Line is designed to operate with shielded, twisted-pair
cable which has a characteristic impedance of 124 ohms. In certain network
configurations, the Local Line must be biased to raise its noise immunity
and to prevent oscillations of receivers connected to the line.
In all configurations, the Local Line must be properly terminated at both
ends of the trunk to prevent an impedance mismatch which could result in
signal reflections on the line.
The need to apply a bias voltage to the Local Line depends upon the
interface configuration of your network.
Three different Local Line media interface combinations exist, based upon
the number and type of interface devices. Possible combinations are:
•Installations consisting solely of self-biased devices.
•Installations consisting of both switch-biased and self-biased devices.
•Installations with PM550 CIMs.
NOTE: This section covers self-biased and switch-biased configurations only.
Appendix A discusses biasing and terminating a Local Line having all three
types of devices.
This configuration exists when all TIWAY I Devices on the network are
self-biasing (i.e., they do not have a bias switch). In this case, the position on
the line of the interface devices does not matter.
This configuration exists when only one TIWAY I Device on the Local Line
has the bias switch, and all other devices are self-biasing. The position of
the switch-biased device on the line does not matter, but its bias switch
must be ON (downward position).
This configuration exists when multiple switch-biased TIWAY I Devices are
attached to the network, and they should be positioned as described in the
following paragraphs.
If two or more devices having the bias switch are attached, place one at each
end of the Local Line, and turn on each bias switch.
All other devices having a bias switch can then be placed anywhere on the
network, with the bias switches turned OFF. See Figure 2-5.
Biasing
Configurations
Networks with only
Self-biased
Devices
Networks with a
Single
Switch-biased
Device
Networks with Two
or More
Switch-biased
Devices