Driven serial interface for reading EEPROM

Background

As a result of some questions concerning my layout of the MAX232 interface circuit for reading the password string from EEPROMs, I have attempted to simplify the wiring of my circuit and document the parts being used. This circuit is based upon Victor Voinea's Driven Serial I2C Programmer, Rev 3.1 diagram. I have used a slightly different layout of this circuit to unlock a couple of IBM 600X laptops but the one I am documenting here will work just as well. If you compare the pictures on this page to the ones that I have posted on my "Recovering an unknown supervisor password from IBM 600X" page you will notice that this version has a few less wires and some "missing" capacitors. The extra wires were only used to place the four wires going to the serial plug in the same area of the breadboard. The extra capacitors were only installed on the breadboard so that the clips I was using could have an easy place to grab onto. Electrically, the two circuits are identical.

Breadboard Basics

I purchased the Elenco 9830C breadboard for this project as it was fairly inexpensive ($6) and small enough for what I needed. Most breadboards have the same structure but variations do exist. Generally, there is a "channel" in the middle of the board which separates the two halves into independent sections. This "channel" is spaced such that an integrated circuit (IC) that is 300 mils (0.3 inch) wide will fit perfectly across the two halves. The MAX232 IC is commonly available in this type of package. Along the outer edges of most breadboards are two, perhaps four, power strips (busses) which run the length of the breadboard and provide multiple points at which to connect devices to either power (VCC) or ground (GND). Refer to the picture below:

Basics of a breadboard

In my case, the Elenco 9830C breadboard has the normal 300 mil center spacing but has four power strips, already marked with "+" (power) and "-" (ground) signs. These power busses are not connected to each other but I'll get to that in a minute. Refering to the picture above, each "column" of five holes on the top side of the center strip are electrically connected together. So are the five holes in the bottom half of the breadboard. But they are NOT connected to each other. I have drawn yellow lines over the image of the breadboard to indicate how each hole is related to it's neighbor. Although I haven't drawn lines over all of the columns, they are in fact connected like the three on the right. The two power busses on the top and the two power busses on the bottom run the entire length of the breadboard. Even though there is a break (i.e. no hole) every sixth position, all of the holes in each strip are connected as shown by the yellow lines. At the other (left) end of the breadboard I have tied these four power busses together in order to simplify the layout of the components. I used a series of wires to bridge the power lines as shown here:

Tying the four power strips together

Circuit Theory

Electrical signals on the computer's motherboard use what is typically called "TTL" levels. These signals are voltage levels which change from ground (i.e. 0 volts) to a voltage level somewhat near 5 volts, thus providing the two binary signals of "zeros" and "ones". The computer's serial port, the 9 or 25 pin connector which is used to communicate with external equipment such as modems, uses a totally different signal format (formally called RS-232) to represent the same "zero" and "one" digital signals. That signal format uses positive and negative voltages, usually +/- 12 volts. The purpose of the MAX232 IC is to convert the TTL levels to RS-232 and RS-232 levels to TTL. The MAX232 normally requires about 5 volts to operate, thus the requirement for the three AA batteries (4.5 volts). In order for the MAX232 to generate the +/- 12 volts for the RS-232 signals, an internal circuit called a "charge pump" is used. That circuit requires large, external, capacitors in order to work. That is why the MAX232 uses five 1 microfarad (µF) capacitors. A newer version, the MAX232A, has a more efficient charge pump and only requires the 0.1 µF capacitors.

Parts Description

A brief listing of the parts that go into the interface circuit and some possible problems you may encounter:

The MAX232 IC is obviously the heart of this interface. As mentioned above, there are two versions you can use for this circuit. Be careful that you know which one you have. I used a Texas Instruments MAX232N. It is actually just a standard MAX232. What is the "N" for? It just identifies which package the IC is encased in. "N" = 16 pin, 300 mil, plastic DIP. The version of MAX232 you use directly affects which capacitors you should also be using. To identify pin 1 of the IC there is usually a "notch" on one end of the package (like a semi-circle that has been cut out of the top). With the notch on the left, pin 1 is the lower, left-most pin. Sometimes pin 1 is also indicated by a small hole placed next to it on the top of the package.
Victor's parts list shows the five capacitors to be of the non-polarized variety (ceramic, mylar, polystyrene) since there is no reference to any polarity (+ or -) for each. I would recommend that you stick with these types of capacitors and not use the "electrolytic", or polarized, versions, simply because you don't have to worry about which lead goes where when using the non-polarized capacitors. However, if electrolytics are all you can find, then my diagram below will help you install them correctly.
The two diodes should have a marking on them to indicate which end is the cathode and which end is the anode. You MUST identify the leads and install them correctly for the circuit to operate. The cathode lead of the diode should have a black or white band drawn all of the way around the diode. Again, referring to Victor's schematic, the cathode end of each diode must be connected to pins 9 and 12 of the MAX232 IC. Since I did not have any 1N4148 diodes, I substituted them with 1N914.
The two resistors should have four color bands painted around the body. The standard color code for a 4.7K ohm resistor is YELLOW, VIOLET, RED. The fourth color code indicates the precision or tolerance for the value of resistance. Typical colors are SILVER for 10% or GOLD for 5%. Either one should work fine.

My Simplified Breadboard

In the picture below, I have highlighted the placement of the components of this interface circuit. At each point where a component lead has been pushed into a breadboard hole, I have drawn a yellow circle (except the MAX232 IC). Notes have been added to the picture describing exactly which component is which. The five capacitors have been identified using the same designation as in Victor's schematic diagram (C1 to C5). If you are using electrolytic capacitors, I have also identified which one should be the positive lead by drawing a blue "+" sign next to the positive lead.

Simplified layout of interface board

Good luck and let me know if this helped you successfully complete the circuit or if you have found any errors with it!

Raymond Kawakami
San Jose, CA
E-Mail: r k a w a k a m i AT y a h o o DOT COM

Copyright 2006
Version 1.0 - Released May 14, 2006

All photos were taken by me using a Canon A10 camera and prepared for web posting using Paint Shop Pro 7. You have permission to link to this page but not to claim it as your own.