M Bus Serial Cable Circuit

The MBus, also known as "M Bus" is a single pin bi-directional bus for both transmitting and receiving data between a cellular mobile phone and a host computer. The communication specification is slow at 9600 bps and only half-duplex.

The physical connection includes only two pins for data and ground, and there is no handshake or flow control. Back when the phone was new it took me a day with an oscilloscope to work this out. The serial bus is useful for setting some of the hidden features of the mobile phone; however, it is not capable of flashing the firmware alone. For flashing the firmware, the FBus is required in addition to this bus.

Parts List

M Bus Circuit Operation

This circuit uses the MAX232 IC to interface a cellular mobile phone to a host computer. The physical connection is through the serial port of the computer also known as RS-232. The MAX232 IC is one of those wonderful components that can reduce the component count for a professional competent engineer.

I designed this circuit with a minimal component count; I suppose this is the difference between the professionals and hobbyists. I managed to make it with just eight components, for the cost of a coffee.

Power Supply

After a little reverse engineering and research with an oscilloscope, I managed to figure out that pin 7, which is Request to Send (RTS), was providing a nice strong positive voltage source. This was remaining there when the flashing and communication software were started, hence this became the likely pin to draw power from. The diode D3 is simply there to block negative voltage swings, since serial communications use negative voltage levels as well.

The output from pin 7 -- on my computer -- was +10 V; hence, it required reducing to 4.7 V for the MAX232 IC. This IC can accept a maximum power of six volts; hence, a simple 4.7 V Zener diode and a 47 Ω series resistor clamped the voltage.

You could always use the Zener Diode Series Resistor Calculator if you are new to the component.

This is a very nice and cheap way of clamping the voltage to required levels. You could of course use a 5-volt regulator such as a 7805; however, I would be too embarrassed as it indicates a lack of design flair.

Diode D5

Diode D5 is used to provide loop-back by joining the input pin 10 and output pin 9 of the IC.

MAX232 Support Capacitors

The MAX232 has internal charge pump circuitry to produce the higher voltages for RS-232 communication. It has a built-in inverter, which generates -10 V, and a doubler, which generates +10 V. Therefore, in the textbook circuit, four electrolytic capacitors of value 1 µF are required. However, in order to keep the component count low, I am not using the onboard doubler, as there is already a +10 V source present. Hence, this circuit design operates with just three support capacitors. Pin 1 and pin 3 is for connecting a capacitor for the voltage doubler circuitry, however it is not connected, and instead pin 2 connects directly to the +10 V source available just after D3.

Mounting Board and Recycling

Since this circuit is temporary for updating one phone to give it a new lease of life, the cost is minimized by using recycled components. I used a very small piece of scrap prototyping board and a recycled DB9 female connector. In the following sections of this article, you can see my circuit build where I built the circuits for both FBus and MBus on a single board.

Testing the Circuit

The circuit test does not require connection to the phone. Simply connect it to the COM1 port of the computer and use HyperTerminal to see if what you type on the keyboard echoes back through the diode D5. If what you type echoes back to the screen on your computer, then the circuit is operating. If not, then you need to get the oscilloscope out to troubleshoot which part of your circuit needs adjustment. Many times, the issue is with the serial port settings on the computer. You will need to check COM1 settings in Control Panel, and ensure that it is present and operating correctly.

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