GAI-Tronics MRTI 2000 (No. PL1877A) Microprocessor Radio Telephone Interconnect Installation & Service Manual - Manual (Page 166)

Theory of Operation PL1877A Microprocessor Radio Telephone Interconnect
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Line Interface
The line interface complies with FCC part 68 rules, allowing direct connection to the public switched
telephone network (PSTN). The interface functions are: lightning and surge protection from the phone
line, isolation, line accessing, pulse dialing, ring detection, hybrid, and longitudinal balance. Land-line
audio, from the PSTN (or PABX) enters the PL1877A via P404. L401, L402, S401, S402, and S403
provide surge protection.
Ring detection is afforded by comparators U403c and U403d, generating an input pulse-train to the
HOST processor. This occurs whenever the level of the ring component exceeds the threshold level of
the positive and negative comparators. There it is validated. U402a functions as a differential mode
amplifier, reflecting a high impedance load to the phone line.
Common mode balance is enhanced by proper setting of VR401. Phone line audio, entering U402a, is
reduced in level by a factor of 20.71. From this point the audio passes on to U402b, comprised as a gain
stage of 26DB. U402b also is part of the hybrid. The receive audio is then injected out of phase with the
line audio at the summing junction of U402-9. The signal at HYBR_OUT (U402-8) is thus suppressed of
the receive audio component.
Phone audio enters the module and into U402b, the line driver. Amplified audio, coupled through T401
and C401, provides isolation and impedance matching to the line. No dc current flows through T401, but
rather only the ac component (audio).
DC loop current flows instead through the constant current circuit comprised of Q401-Q405 and
associated components. Opto-switch U401 serves as the access relay, and provides isolation to the phone
line from high voltage surges. The opto-switch is also pulsed at the selected dialing rate by the
microprocessor if pulse dialing is required.
Optional Line 2 Interface
The optional line 2 interface provides the ability to answer and originate calls on a second phone line.
Dual ring detect is provided by U502, necessary to detect ring on line 1 when line 2 is accessed. U502a-
d is configured as two, dual polarity comparators. Line selection is provided by energizing relay K501,
under processor control via LIN_Sel. Land-line audio is then routed through the same path that line 1
used (T401). From there the operation is identical to line 1.