Nortel Networks 411-2021-111 manual DC coupled signals

5-8Power and Grounding Requirements

DC coupled signals

DC coupled signals are considered undesirable from a grounding point of view for the following reasons:

•If a signal is routed to another system on a separate ground, then isolation is lost due to a connection via the signal return.

•Any noise on the system ground can resistively couple onto the signal potentially causing degradation in system performance (for example, bit errors on digital signals or unwanted noise pick-up on analog signals).

The Metrocell contains the following DC coupled signal links:

•TRU terminal interface (RS-232 data only) — This potentially creates a connection between the system ground and the AC ground in which the connected terminal can affect system performance and damage equipment. A RS-232 opto (for example, Telebyte model 268) is recommended for this connection and this link should only be used in commissioning or doing maintenance and not be connected in normal operations.

•Control signals between the TRU and DPA (TTL/COMS logic levels) — These signals are restricted to the shelf backplane only.

•Alarm signals between the ATC shelf and the TRU/DPA shelf (+27 V) — These signals are restricted between the two shelves on the Metro RF Frame which provides a good low resistance ground to frame.

•Interframe alarm signals (+27V) — These signals are actually opto- isolated at the receive end (that is, at the ACU). The return path is through the system framework ground.

•ATC remote interface (RS-232 or RS-485) — (Future Development.)

411-2021-111 Standard 01.01 June 1996