Technical description 7000 series TouchLine station

The 7000 series of TouchLine master stations are loudspeaking intercom stations, with or without handset, and with or without display. In this document, the full-featured stations with handset and display are describes. These are the 70013 desk station and the 70343 eall/desk stations, wich functionally are the same.

Functional Description

For the description of the station, refer to circut diagrams and block diagrams.

The station is connected to the exchange by 4 wires; a-, b-, c-, and d-wires. The a/b-wires are the loudspeaker wires, and the c/d-wires are the microphone and digit selection wires. The currents in the a/b-wires when display module is installed is shown in parantheses.

Functions of the a- and b-wires

Loudspeaker

The loudspeaker is connected in series with a volume control and a reed contact, witch disconnects the loudspeaker when the handset is lifted. A capacitor prevents DC currents from flowing through the loudspeaker. A high-value resistor keeps the capaciror charged to avoid clicks when the handset is replaced.

Handset operation

This is a DC current control circuit consisting of a 6K8 resistor in series with a reed contact. Both reed contacts are situated in the base part of the station and are kept close by the built-in magnet in the handset. When handset is in place, the current in the a/b-loop is approximately 3 mA (13 mA). When handset is lifted, the current is reduced to zero (10 mA). This is a signal for the exchange to:

• Reduce the central amplifier gain by 10 dB to compensate for higher michrophone level.
• Accept an incoming call when station is in private.
• Prepare for automatic disconnection when handset is replaced.
• Prepare for open duplex function when handset is lifted at both sides.
• Mark the station busy while the handset is lifted.

Handset loudspeaker circuit

This consists of a small loudspeaker witch is connected in series with a DC-blocking capacitor and a series resistor to reduce power to the loudspeaker.

M-key circuit

When pressing the M-key, the current in the a/b-loop is increased to approximately 10 mA (20 mA). A speed-up circuit will give a short pulse with higher current so that the detection of the M-key in the exchange is faster. This is necessaty for the operation of the function keys, witch activate M-key simultaneously with transmission of digit. By shorting a solder bridge (S4), the depressing of an M-key will activate an ID-signal in c/d-wires. This is used for the Program Conference feature.

The activation of an M-key is a signal to the exchange to prform one of the following function:

• Manual control of speech direction in a conversation.
• A short M-key push will re-establish the voice switched (duplex) mode.
• Accept an incoming call when station is in private.
• Allows replacing handset without disconnecting call.
• Remote control function (e-wire control)

C-key circuit

When depressing the C-key, the current in the a/b-loop is increased to 20-25 mA (30-35 mA). This is a signal to the exchange to cancel the conversation of to interrupt the number selection.

Display module

A display module may be connected betwen the a/-wires. The DC current drawn by the module is a constant 10 mA. Data to the module is transmitted as a frequency shift signal, with 46.9 kHz representing mark and 62.5 kHz representing space. A further description of the display module is given later.

Functions of the c- and d- wired:

Microphone preamplifier

The michrophone preamplifier is a two-stage transistor amplifier with DC feeding of the microphone, which is an electret condenser michrophon. The amplifier is only operative when the c-whire has a positive voltage referred to d-wire. This is the case when the speech path is set up through the exchange. The light emitting diode (LED), which is connected in series with the amplifier, is thus the direct "live microphone" indication. A third transistor will cut off the current in the amplifier when a digit key is depressed during conversation.

Digit transmitter

The digit transmitter is a digital IC that produces sinewave frequencies by a rasistor network. The oscillator frequency is divided by divided by a number dependence on the key depressed, producing a frequency 16 times the audio frequency. This frequency will then activate the outputs to the resistor network in sequence so by appropriate selection of resistors, a sinewave is produced.<br\> The frequencies produced are approximsately:

Digit 0:	500 Hz
Digit 1: 700 Hz
Digit 2: 900 Hz
Digit 3: 1100 Hz
Digit 4: 1300 Hz
Digit 5: 1500 Hz
Digit 6: 1700 Hz
Digit 7: 1900 Hz
Digit 8: 2100 Hz
Digit 9: 2300 Hz