Nokia 3310 Parallel Flashing Cable

The FBus is a two-pin communication system (through a parallel port) used in cellular mobile phone systems for flashing the firmware. Also called a "Dejan" cable, and a "Service" cable, its purpose is to update the firmware of the Nokia series of mobile phones. This bus specifically reads and writes the firmware onto the flash memory chip and is independent of the MBus. However, for flashing the firmware, it works in conjunction with the MBus, and therefore both the MBus and FBus connections are required for flashing purposes.

The physical layer of the FBus consists of three pins, Tx, Rx, and a common ground. The Nokia 3310 has a 2 MB Atmel flash memory IC that is read and written to by this bus over the parallel port LPT1. The voltage level of the data signal is typically around 2.8 V.

Building the FBus Service Cable Circuit

This circuit uses the 74HC14 IC, which consists of six inverting Schmitt trigger buffer circuits. An advantage of implementing Schmitt triggers is that is has a way of sharply defining digital signals making them jitter-free. However, the real secret reason is that it is my favourite IC because you can get twenty of these on eBay for less than the cost of a cup of coffee.

The 74HC14 is a versatile IC, and you can make almost anything with a couple of these. I always say that if I had a bag of these linear integrated circuits, I could build the USS Enterprise.

Parts List

I managed to keep the component count down to five readily available components. A parallel port connector D25 is very expensive these days, and I managed to use a recycled one from a computer cable.

LPT1 and FBus Voltage Levels

A computer parallel port such as LPT1 operates at TTL voltage levels. Although the precise threshold voltage levels may vary depending upon the computer, generally a voltage between 2.4 V and 5 V is a binary 1, and any voltage lower than 0.8 V is a binary 0.

Therefore, the parallel port recognises communication with signal levels as low as 2.8 V as it is well within the threshold parameters. This voltage level is also compatible for the FBus, therefore communication has to occur at this level between the phone and the computer.

Voltage Level Translation

Since the parallel port at the computer end uses TTL voltage levels of 5 V, and the FBus uses CMOS voltage levels of 2.7 V, a cheap method of voltage level translation is required.

The 74HC14 IC is very useful for voltage level translation because its logic levels follow its power supply level. The transfer characteristics of this IC provide reliable gate switching action with Vcc as little as 2.8 V. Therefore powering it with 2.8 V will mean that its output logic level will be 2.8 V for binary 1.

This is a very elegant and cost effective way to translate voltage levels using a very low cost readily available IC. You could of course over-engineer this with transistors, voltage regulators, and expensive components, but it is not necessary, unless you have more money than sense.

Power Supply

The power for the circuit comes from pin 3 and pin 5 on the parallel port. Usually, the purpose of these pins is not to supply power; however, the power requirement is very small for this CMOS IC.

D1, D2 are ordinary 1N4148 diodes. This together with a 4.7 kΩ resistor ensures that Vcc remains fixed to 2.8 V. The 100 nF capacitor provides decoupling and should be close to the power pins of the IC. When Vcc is 2.8 V the data signal from the chip will also be around 2.8 V, which is compatible with the FBus.

Nokia 3310 Data Connector

A special connector, which connects to the phone behind the battery, is ideally required; however, I could not find any. Since this was a one-off one-time project to update the firmware, it was not logical to purchase an expensive connector from a specialized dealer.

Since there are only four connections, I was thinking of using Blu-tack and some sharp pins to secure a temporary connection to the port. The other alternative was to solder temporary wires directly to the PCB. In the end, I decided to install a very thin flexi cable as shown in the port modification page. I managed to solder a thin ribbon cable directly to the port and use it with a different socket. For some of you it will take a few days, and the rest the terms of your natural lives.

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