Nokia 8110 Cellular Data Suite Interface

Here is the schematic for N8110i, N8110 and N3110 phones. If You want it in better resolution, is here.

All interface is powered by RTS and DTR signals. D1 and D2 diodes I recommend in Shottky design since lower dropout.So 5V regulator should be in "L" design (low drop version). You can use normal diodes and normal voltage regulator, but MAX232 may be powered by voltage less than 5V (depends on COM port in computer - log. 1 on RS232 may be between cca +5V and +15V, but in notebooks it is on lower value, on diode is drop about 0.6V, so before regulator it may be 4.4V and in this case after regulator about 4.1V) - it can be wrong (I tested MAX232 works fine on voltage 3.5V), but amperage of COM port will be unnecessarily bigger.

Zener diodes DZ1 and DZ2 are VERY important to safety of Your phone. Normally are values of log. 1 about 5V (or 3.3V ??) and little overcross of this value can damage the phone - so power blast at turning on computer or connecting interface into computer. Voltage 5.1V will damage phone port - tested :-((( !!! DZ1 and DZ2 diodes cuts-down this voltage to 3.3V - it is enough to log. 1 and it is hardly under maximal voltage value (or exactly fine, if log. 1 is 3.3V ???).

In all, it is standard wiring of MAX232. Pins No.8 (R2IN) and No.9 (T2IN) You needn't to connect to ground.

Here is phone connector description, the same in better resolution and with signal description here.

Here is picture of 2-SIDED printed wiring, the same in better resolution. This variant is for direct connecting to Cannon DB 9 female connector, see pic.

If You want 1-SIDED printed wiring, (here in better resolution), You will have "bubble" in the centre of the cable, see pic.

Only warning, pictures ARE NOT in 1:1 scale, so it is necessary to redraw them. If You want originals (they are in the right scale) in CorelDRAW 8, are here.

Communication protocol