Specifications

155
Dual Trunk Card (702-9117) Rev D and Later
PABX extension or C.O. End-to-End service. The E&M position is typically for PABX
trunks using E&M, 2-wire audio signaling. In the GND ST position, the interface is for
PABX ground start trunks.
Two detectors are used on each of the two telephone lines: a loop detector and a ring
detector. The ring detectors are used when in the End-to-End configuration. The loop
detectors are used with loop-start, ground-start, and E & M lines. R57 and R59 set the
threshold of the Trunk A loop detector. R64 and R66 set the threshold of the Trunk B loop
detector. The rest of the telephone line control consists of a current-limited 48-volt power
supply; DS15, DS16 and C2 for Trunk A, DS13, DS14 and C3 for Trunk B, and relay
control.
Full-duplex hybrids separate the incoming and outgoing audio. The circuitry effectively
cancels out the outgoing audio from the incoming audio. Part U26 for Trunk A and U27
for Trunk B form inductor and variable capacitor simulators to model the impedance of
each telephone line and transformer. Each trunk is provided with two adjustments: R
Balance and C Balance. These balance controls are used to “match” the hybrid to the
telephone lines. The better the match, the better the isolation provided.
Trunk A incoming audio is amplified by U28 and level adjusted by R17. Trunk B
incoming audio is amplified by U4 and level adjusted by R9. The amplified outputs are
then fed into the Envelope Detector, AGC, Modem, and DTMF decoder for each trunk.
Trunk A's Envelope Detector consists of U28 and associated components. The voltage
across C53 is connected to an analog input of the microprocessor U14. Trunk B's
Envelope Detector consists of U4 and associated components. The voltage across C12 is
connected to an analog input of U14, as well. The detect threshold is a programmable
parameter due to this use of the microprocessor's analog to digital inputs.
The AGC circuits for trunks A and B use their respective envelope detector outputs. Each
is comprised of a FET and a peak detector. The varying source-drain voltage of the FET
introduces distortion which is minimized by feedback from U28 (U4 for trunk B). The
peak detector's threshold determines the nominal peak to peak output level the AGC will
attain. The output of each AGC is applied to the PCM Combo CODECs, U8 for Trunk A,
and U5 for Trunk B.
The paging software communicates with the Dual Trunk Card microprocessor through the
dual-port RAM U35. The RAM appears in the I/O map of the system when the card is
“selected”. U33 and a portion of U37 make up the select circuitry. Each card has a unique
select address determined by the setting of the DIP switch SW2. The card becomes NOT
selected when any other card is selected; thus, only one card is present in the IBM I/O
map. The reset line of the microprocessor can be controlled by the IBM bus via a portion
of U37. SW1 is a push-button switch that creates a non-maskable interrupt used for testing
purposes.