User's Manual

5-8 Power and Grounding Requirements
411-2021-111 Standard 01.01 June 1996
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.)