PM550 CIM Requirements
A-4 SIMATIC TIWAY I Series 500 NIM User Manual
A.3 Local Line Bias and Termination
For TIWAY I networks using the Local Line, some devices are provided with
a toggle switch to allow selection of bias or no bias for the Local Line. Other
devices have self-biasing built in and therefore have no Bias Switch. The
CIM is provided with a jumper which allows selection of bias or no bias.
The Siemens Local Line must be properly terminated at both ends of the
trunk to prevent a possible impedance mismatch that could result in signal
reflections back along the line.
Termination is required regardless of the numbers or types of devices
attached to the network.
NOTE: All Siemens tap housings contain factory-installed termination
resistors. If the Tap Housing is not used to terminate the Local Line, the
terminating resistors must be removed when the output cable is attached.
On networks using devices with the Bias Switch, CIMs, and self-biased
devices, a bias must be applied to the line using the switch-biased device as
described in the previous section. All CIMs on the network should be
positioned somewhere between the two outermost switch-biased devices.
Terminate the network according to the instructions in Chapter 2.
On networks using devices without the Bias Switch, bias must be applied to
the Local Line by the CIM. To accomplish this, proceed as follows:
1. On networks having only one CIM, place this CIM at one end of the
Local Line trunk, and install the jumpers provided with the CIM in
accordance with the instructions in the PM550 CIM User’s Manual,
Manual Number PM550–1101154–4. This will apply the correct bias to
the Local Line to prevent oscillations of the CIM receivers.
To terminate the Local Line, replace the existing terminating resistors
inside the CIM T-Tap housing. Connect 150-ohm resistors between the
LLM+ and the cable shield, and also between LLM– and the cable
shield. This resistance value is 150 ohms in each instance, and the
connection is made in accordance with instructions inside the tap
housing. The combination of termination resistors in the CIM, together
with these 150-ohm resistors, provide the correct Local Line
termination. Then, terminate the other end of the local line as
described in Chapter 2.
Networks with
Switch-biased
Devices and CIMs
Networks with Only
Self-biasing
Devices and CIMs