Caller Display
==============
One advantage of the introduction of the digital exchange to the telephone
network is that the service provider (for example, British Telecom in the UK)
can offer a number of interesting data services to enhance the usefulness of
their network.

One such service is the CLID service, or calling line identity, where the 
number of the person calling you is sent in modulated form just before the
first ring that can be heard on the telephone bell.

Using this caller id is simple and there are many ready made devices available
to demodulate the data, one such device is the Mitel MT8843, which also
interfaces direct to a microprocessor bus. 

Using the BBC micro allows a number of interesting call logging functions to
be performed, or call filtering to avoid certain people contacting you (such
as salesmen), or linking it to a database of your customer's numbers so that
you can give a personalised answer.

PARTS...
The circuit diagram in figure 9 on page 10 of the datasheet in conjunction 
with the schematic shows electrically how to assemble the unit, requiring the 
following parts:
1x  6P4C pin RJ11 telephone jack
1x  4 pin RJ11 to BT line jack lead
1x  150V S07K150 disc varistor
2x  22nF 100V 5% polyester capacitor
2x  100nF 100V 5% polyester capacitor
4x  1N4003 diode
6x  1N914 or 1N4148 diode
2x  430K 5% resistor
2x  499K 5% resistor
2x  34K 1% resistor
1x  53K6 1% resistor
1x  60K4 1% resistor
1x  464K 1% resistor
1x  200K 1% resistor
1x  301K 1% resistor
1x  150K 5% resistor
1x  220nF ceramic capacitor
1x  3.579545MHz crystal
3x  100nF ceramic capacitor
1x  100K 5% resistor
2x  422K 1% resistor
1x  20 way IDC user port connector
Length of 20 way ribbon cable

BENEFITS...
*Runs off 5v supply from the user port
*Log and process incoming calls limited only by memory
*Simple user port interface has interrupt capability

BUILD...
1.Build the circuitry per the diagram in the datasheet
2.Change the mode to one, by tieing pin 9 to +5v instead of ground as shown
3.Simply split the 20 way ribbon cable and connect as suggested in the 
  schematic diagram
4.Crimp the IDC plug on the other end of the ribbon cable and connect to the
  BBC micro's user port.
5.Attach the phone jack to a secondary socket in your house. Note that the
  circuit you have built isn't BABT approved, and you may be liable for any
  damage caused to your service provider's equipment, check your wiring
  carefully as everything you do is at your own risk.

HINTS...
*The application circuit shows a TISP4180 or equivalent wired across the line
 to prevent surges on the network (eg.a nearby thunderstorm) from damaging
 the circuit. The easy to obtain 150V varistor can be used in its place.
*Some of the resistor values are quite hard to achieve with just one part
 unless an E96 range is chosen. They can be made up from two parallel or two
 series resistors instead.
*A 6P4C RJ11 connector is called this because it has 6 positions but only 4
 connections, the middle 4 have pins in them, the outer two are just spacers.
 In which case for a BT cable, "A" is pin 2 of 6, "B" is pin 5 of 6.

FROM...
Robert Sprowson.
Happy soldering!