NuukiaWorld project:
FBUS & MBUS adapters

5. Dual-mode adapter

5.1 12-component FBUS & MBUS dual-mode adapter

This chapter contains building instructions for NuukiaWorld's 12-component FBUS & MBUS adapter based on the idea described in chapter 1. The circuit is a combination of the 8-component FBUS-only and MBUS-only adapters, so you should check also the information on those pages. The mode switching is automatic because the switch shown in the general dual-mode adapter in chapter 1 is replaced with a field-effect transistor switch.

CAUTION: This dual-mode circuit is very simple and many not work with all PCs. Better operation may be obtained by using the single-mode adapters (or the super miniature dual-mode adapter).

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.

Figure 5.1 NuukiaWorld's dual-mode adapter using MAX 232.

Table 5.1 Required components for NuukiaWorld's 12-component dual-mode 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 a pull-up to pin 10, smaller capacitors and zener diode. V+ must not exceed 7 volts.
  1 16 pin IC socket 0.30    
D1,D2,D4 3 1N5819 1.41 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    
R2 1 470 kohm 0.05    
R3 1 10 kohm 0.05    
Q1 1 BF245 0.64 BF244 (different pin arrangement) or any other symmetric N-channel JFET  
J1 1 DB9 female 0.34 DB25 female (different pin arrangement, see chapter 2.3)  
J2 1 Phone connector      
    Copper-striped circuit board 13 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 with modified power supply part and FET-based soft switch between the FBUS and MBUS modes. See the 8-component FBUS-adapter description for details of the power supply.

The schottky diode D4 is used to make the R2OUT open-collector compatible. As described in the chapter 1, there is an internal pull-up resistor in the phone MBUS pin.

The mode selection is made with the RTS line. When the RTS is at -12 volts, the channel of the FET Q1 disappears and the FBUS Rx and Tx lines are separated from each other. If the RTS is put to a positive voltage, the channel of Q1 is opened and the pin 10 (T2IN) of the MAX232 is connected to the MBUS pin of the phone. The resistor R3 prevents the short-circuit effect of simultaneous data bursts on the MBUS and FBUS Tx lines.

5.2 Building the dual-mode adapter

The circuit is so simple that a printed circuit board is not required (see figure 5.2). The DB9 (female) connector is soldered directly on the edge of the board. (The five pins of the connector shown in figure 5.2 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.) The pin 7 (RTS) is wired to the circuit board separately. Remember to short-circuit pins 1 and 2 of the connector as shown in figure 5.2.

Figure 5.2 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 three diodes (D1, D2, and D4) in the circuit are schottky ones to avoid unnecessary voltage drop.

CAUTION: If the FET type you are using as Q1 is not exactly the BF245, you have to check the correct S, D, and G pin order for the particular type you are using. There is no standard pin arrangement for FET transistors, even if they are in the same packages and look similar.

Use an IC socket for MAX232. Please note that in the circuit board layout shown in the figure 5.2 the pins 1, 3, 11, and 14 of the IC socket must be cut away.

5.3 Testing the dual-mode circuit

The testing of this dual-mode adapter is similar to the testing of the other adapters. See chapter 3 for details. Do not connect the phone to the adapter before all tests are passed.

With some terminal programs connecting Tx & Rx line together is not required in the loop-back test. If the terminal program asserts the RTS line, then the loop is automatically formed and no external connections are needed. If not, then the Tx & Rx must be connected as told in the loop-back test procedure. (In other words, if the RTS is at a positive voltage, Q1 should connect the Rx and Tx together automatically.)

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

Note: At least most of the versions of LogoManager initialize the PC serial port badly in the start-up. If the particular COM port has been set to hardware handshaking before LogoManager use, LogoManager switches a dual-mode cable into the MBUS mode. True Nokia-compatible applications holds the RTS in -12 V for the FBUS mode.

- © Copyright 1998, 1999, 2000, 2002 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.

1.2.2002 - nuukiaworld@gmail.com
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