NuukiaWorld project:
FBUS & MBUS adapters

6. Dual-mode adapter with MAX3221

6.1 Super miniature FBUS & MBUS dual-mode adapter

This chapter contains building instructions for NuukiaWorld's super miniature FBUS & MBUS dual-mode adapter based on the idea described in chapter 1. The circuit is based on Maxim's smallest single-supply RS-232 transceiver chip MAX3221CAE. The size of the circuit board is then so small that it can be easily mounted in a normal plastic DB9 connector box. This makes the adapter cable very professional-looking.

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. 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.

CAUTION: The builder must be familiar with the surface mount soldering methods to build this super miniature adapter. Soldering the SSOP package may be impossible without special SMD soldering tools. The availability of the components may be poor at regular component dealers.

Figure 6.1 NuukiaWorld's super miniature dual-mode adapter using MAX 3221CAE.

Table 6.1 Required components for NuukiaWorld's super miniature dual-mode adapter.




Component size


U1 1 MAX3221CAE SSOP Any MAX3xxx series single-supply transceiver, for example MAX3232. Requires different PCB layout. Unused transmitter inputs "TxIN" on the chip must be tied to VCC.
D1,D2,D4 3 LL103A 1206 Any schottky diode
D3 1 BZV55C3V9 minimelf Any zener diode from 3.3 V to 4.7 V
C1,C2,C3,C4 4 100nF 0603 or 0805  
C5 1 1µF 1206  
R1 1 47 ohm 1206   
R2,R4 2 402 kohm 1206 Any resistor between 300 kohms and 500 kohms
R3 1 10 kohm 1206  
Q1 1 SST310 SOT23 Any other symmetric N-channel JFET (check the pin arrangement)
J1 1 DB9 female   DB25 female (different pin arrangement, see chapter 2.3), require different PCB
J2 1 Phone connector    
    One-sided circuit board 15 x 27 mm2

Note: Parts substitutes marked "Equivalent*" may be used in the circuit without any problems. Using components with different size or package may require modifications in the PCB layout.

The adapter design consists of a standard MAX3221 circuit with the external connection based on results of the chapter 1. The FET-based soft switch allows the adapter operate between the FBUS and MBUS modes.

The three diodes (D1, D2, and D4) in the circuit are schottky ones to avoid unnecessary voltage drop. 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 FET Q1 is disconnected and the FBUS Rx and Tx lines are separated from each other. If the RTS is put to a positive voltage, the channel in the Q1 is created and the pin 11 (T1IN) of the MAX3221 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.

6.2 Building the super miniature dual-mode adapter

To make the circuit so small that it can be installed in a plastic DB9 connector box a printed circuit board (PCB) is required. The layout is shown in figure 6.2.

This circuit fits in a plastic DB9 connector box if the locking screws are not installed. The strain relief can be still used for the phone cable. See picture 8.7 for the type of the connector case.

Figure 6.2 Printed circuit board layout for one-sided board (300 ppi). To print the layout in scale, use an image processing application and ensure that the printing will happen exactly at 300 pixels per inch. The layout shown is a mirror image; the texts must be readable on the board. (Download Postscript file.)

Note: The layout in figure 6.2 is not optimized for reflow soldering methods.

Component placement is shown in figure 6.3. Note that there are some unused component places (see chapter 7 if you are wondering why).

Figure 6.3 Component placement for the circuit board layout shown in figure 6.2. (This is not mirrored.)

The DB9 (female) connector is soldered directly on the edge of the board. (The five pins of the connector are soldered onto footprints. 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 hole "RTS" with a wire.

The phone cable is connected to the holes "M2" (MBUS), "RX" (FBUS Rx), "TX" (FBUS Tx), and "GND".

6.3 Testing the miniature 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 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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