NuukiaWorld project:
FBUS & MBUS adapters

3. FBUS-only adapter

3.1 8-component FBUS-adapter

This chapter contains building instructions for NuukiaWorld's 8-component FBUS-only adapter based on the idea described in chapter 1. The industry standard MAX232 has been used as the RS 232 buffers. The MAX232 chip is a very common 16-pin that is manufactured by many IC manufactures with different codes.

This adapter may be used with all Nokia phones with both FBUS and MBUS. The voltage level of the phone is not a problem, so this circuit is useful also with older phones than the Nokia 5110/6110 series. For simplicity, this page uses the pin outs of the Nokia 5100/6100 series GSM phones. You must find out the correct pin numbers of the phone by yourself, if you are going to use another phone model.

Note: For example, the Nokia Data SuiteTM application uses only the FBUS transmission method. If you are building an extra cable for your NDS, please prefer the easier FBUS-only circuit described here.

Figure 3.1 NuukiaWorld's FBUS-only adapter using MAX 232.

Table 3.1 Required components for NuukiaWorld's 8-component FBUS adapter.




Price* ()


Emergency solution***

U1 1 MAX232 0.69 Other manufacturer's equivalent substitute (possibly all "232" chips are not equivalent with MAX232) MAX3232, requires pull-up to pin 10, smaller capacitors and zener diode. V+ must not exceed 7 volts.
  1 16 pin IC socket 0.30    
D1,D4 2 1N5819 0.94 Any schottky diode 1N4148
D3 1 4.7V 400mW zener 0.12 Any 4,7 volt zener with higher power rating 78L05-based linear regulator circuit
C1,C2,C3 3 1µF 16V 0.30 Any 1µF capacitor with higher voltage rating  
R1 1 47 ohm 0.05    
J1 1 DB9 female 0.34 DB25 female (different pin arrangement, see chapter 2.3)  
J2 1 Phone connector      
    Copper-striped circuit board 11 x 10 holes

Note: The "Price*" is an example price (that NuukiaWorld had to pay in Finland) for the total number of the component when bought one by one. Tax (VAT) is not included. Parts substitutes marked "Equivalent**" may be used in the circuit without any problems. Parts marked "Emergency solution***" are not recommended and may require other modifications to the circuit. These modifications require good knowledge in designing electronic circuits, may affect the reliability of the circuit, and such circuits are not tested by the NuukiaWorld. Try to avoid those components.

The adapter circuit consists of a standard MAX232 design. The interesting part is the power supply, where the positive voltage rectified from the DTR signal is directly used as the V+ of the MAX232. Therefore the other of the internal charge pumps is disabled and the number of capacitors is reduced. [Maxim1997]

The main goal in this power supply design is to ensure better voltage level on the VCC pin. The V+ is now taken directly from the rectifier and the V- is generated with the internal charge pump of the chip using capacitor C2. The current in VCC pin is then reduced and the voltage there is enough to be in specifications, at least 4,5 volts.

The pull-up resistor from the basic configuration of an FBUS adapter shown in chapter 1 has been removed, and instead of it the internal 5 kilo-ohm pull-up of the MBUS pin has been used. (The internal pull-up pulls to +3 V, which is not available in the adapter circuit at all!) Although the FBUS data now enters also to the MBUS input, it creates no problems.

The T2IN input the MAX232 has an internal pull-up of 400 kilo-ohms, so the high impedance state of FBUS Tx is ok.

3.2 Building the FBUS-only adapter

The circuit is so simple that a printed circuit board is not required (see figure 3.2). The DB9 (female) connector is soldered directly on the edge of the board. (The five pins of the connector shown in figure 3.2 are on the copper side of the board. The board is pushed into the gap between soldering lugs 1...5 and 6...9 of the connector.) Remember to short-circuit pins 1 and 2 of the connector as shown in figure 3.2.

Figure 3.2 Circuit board layout for copper-striped board (component side).

The capacitors could be almost any kind according to Maxim, and I used 1 µF 35 V tantalum ones because of their small size. The voltage rating of 16 volts is enough. The two diodes in the circuit are schottky ones to avoid unnecessary voltage drop.

Use an IC socket for MAX232. Please note that in the circuit board layout shown in the figure 3.2 the pins 1, 3, 11, and 14 of the IC socket must be cut away.

Please note that this circuit uses the internal pull-up of the MBUS pin of the phone for pulling up the FBUS Rx line. The connection to the MBUS pin is therefore required, even this circuit does not function as an MBUS-adapter. If you are going to use this adapter with a Nokia 5100/6100 series phone, you must connect the FBUS Rx & MBUS to pins 6 and 7, FBUS Tx to pin 8 and GND to 9 in the connector of the phone. See chapter 2 for pin arrangement.

The connection to the phone is the hardest step. The easiest solution is to find an inexpensive headset with all pins connected and cut the connector from it. Check that the connector is openable.

3.3 Testing the FBUS-only circuit

After you have built and checked the circuit (the polarity of the capacitors), test the circuit. Do not connect a phone to the adapter.

- 3.3.1 Testing the power supply part

  1. Connect the adapter to computer. Do not insert the MAX232 yet into the socket.
  2. Open HyperTerminal (or equivalent terminal program) and set the device to corresponding COM port and set flow control to "none". Type some dummy characters to the terminal.
  3. Measure the voltage between the pins 15(-) and 2(+) of the IC socket. It must be more than +5 volts. Use a normal digital volt meter (DVM) or digital multimeter.
  4. Measure the voltage between the pins 15(-) and 16(+) of the IC socket. It must be about 4.7 volts.
  5. Choose Call - Disconnect from the menu. The voltage may disappear, but not with every computer.
  6. Insert the MAX232 (check the direction). Type several characters to the terminal to switch the power back on. Check the voltage of pins 2 (hopefully more than +5 V) and 6 (negative voltage, hopefully more negative than -5.5 V, but -3 V is usually enough) of the chip. Use pin 15 (GND) for the - probe.

If you get about the same results in these measurements, the power supply of the adapter is possibly working correctly. Proceed to the loop-back test.

- 3.3.2 Loop-back test

  1. Open HyperTerminal (or equivalent terminal program) and set the device to corresponding COM port and set flow control to "none".
  2. Choose Call - Disconnect from the menu. (If not already disconnected.)
  3. Connect the FBUS Tx and FBUS Rx (pins 7 and 8 in the Nokia 5110/6110 connector) of the adapter together.
  4. Now if you type characters to the terminal, they will appear on the terminal window. (The adapter successfully converts the RS 232 data to 3-volt signals and back.)

Note: The HyperTerminal program supplied with the Microsoft® Windows 95® and Windows NT® operates usually rather badly (if operates at all). If you have problems with the tests, please try another terminal program.

If you do not get the correct results in these measurements, there is a failure in your circuit. Check the circuit carefully and retry the tests.

- 3.3.3 Nokia Data SuiteTM test

To test the FBUS operation with Nokia Data SuiteTM 2.0, use the function "Contacts directory". If there are problems with the FBUS communication, the message "Failed to read entries from the phone" occurs easily. A cable may work fine with the other functions and applications, so testing with the "Contacts directory" is strictly recommended.

- © Copyright 1998, 1999, 2000 Panu M Tuominen. All rights reserved. Republishing this document partially or completely is forbidden. Internet hyperlinks to the main page are allowed and such linking is encouraged. You are doing everything at your own risk. See the WARNING! in the general information page.

For example, Nokia 5110 and Nokia 6110 are 5100/6100 series phones. The tests have been done with Nokia Data Suite 2.0, Nokia Logo Express, LogoManager demo version, and a Nokia 6110.

21.5.2001 - nuukiaworld@gmail.com
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