Instruction manual

FUNCTIONAL DESCRIPTION
● Port Circuits
Each of the eight identical port circuits allows the connection of interface equipment
having an RS-232 compatible serial interface to the switch. The circuit provides
asynchronous full duplex data transport at standard speeds from 300 to 19,200 bps
and a low data rate (<300 bps). Each port includes an Asynchronous Data Unit
(ADU) to extend the serial communications link length and provide safe isolation.
The ADU terminates to another ADU at the Customer Provided Equipment (CPE).
The distance between the digital switch and CPE is inversely proportional to the
speed at which the link is run.
Throughout the circuit,
various gates are used to provide a means of isolating
devices for automated circuit pack testing.
Typically, these devices are crystal
oscillators or memory components attached to the microprocessor bus.
DID Trunk (TN753)
The DID Trunk CP (Figure 3-11) interfaces eight central office trunks arranged for Direct
Inward Dialing (DID) and the TDM bus. The DID Trunk has the following unique circuitry:
● Port I/O Circuit
This circuit consists of bus expanders for communication between the on-board
microprocessor and the port circuits.
It receives commands from the on-board
microprocessor and distributes them to the individual port circuits. It also accesses
the port circuit scan points and passes the information to the on-board
microprocessor.
● Port Circuits
The eight port circuits are identical.
Each port circuit consists of a codec, balance
network, trunk interface unit, and loop termination circuit.
The codec is a 4-wire circuit that converts the NPEs output to an analog signal.
Likewise, it converts the analog signal from the Central Office (CO) to a PCM signal
to the NPE.
The trunk interface unit contains a hybrid, a 2-wire interface circuit, and control
circuitry. The hybrid circuit converts the 4-wire analog signal from the codec to a 2-
wire analog signal that is connected to the analog line by the 2-wire interface circuit.
The control circuitry controls loop current, internal signal gain, terminating resistance,
battery feed shutdown, and battery reversal.
The circuit pack accepts dial pulse
signaling.
The loop termination circuit provides a fixed impedance to the DID trunk.
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