Service Forum

[AIM]

Hello,
Could anyone by any chance help me with a strange problem with the driven circuit breadboard I made? The eeprom reader software doesn't report an error, but only reads in zeros, regardless whether the circuit is attached to the eeprom or not.

Powering off the circuit, the software reports a circuit or bus error, so I know the circuit is being somewhat detected.

Having read as much of the forum as I could, I believe I read somewhere that the voltage on pin 2 should be at least +8V, and pin 6 to be at least -8V for the program to operate correctly. However, the reading for pin 2 is +9.56V, and pin 6 is -4.73V (approximately half that of pin 2). Removing the capacitor across pins 4 and 5 gives readings of +9.87V (pin 2) and +0.53V (pin 6), so I know that the capacitor still has an effect.

Removing the capacitor on pins 1 and 3 gives voltages of +4.52V (pin 2) and -2.26V (pin 6), with the other capacitor replaced.

I have replaced the MAX202CPE several times (although I understand that this is the same as the MAX232), and the capacitors for pins 1&3 and 4&5 several times, I find it hard to figure out what the problem is. I have already checked the circuit connections several times now, against Victor's circuit plan, and one published by Maxim. Here's a summary of the important parts:

IC: MAX202CPE
Capacitors: 1 micro farad electrolytic
Power supply: USB port powered
Built on a breadboard
Laptop: Thinkpad 600x (also have a 600E to fix as well)

Any help would be appreciated. Thanks.

[rkawakami]

It appears like the MAX202CPE is equivalent to the MAX232A version. This means that the capacitors should be 0.1uF, not 1.0uF. This is the datasheet I found on the 202CPE:

http://www.chipdocs.com/pndecoder/datasheets/MAXIM/MAX202CPE.html

By the way, have you seen my easier way to get to the SDA and SCL signals for the 600X?

[AIM]

Well, I've replaced all the capacitors with 0.1 uF, but the problem still persists.

Yes, thanks for the easier method for the Thinkpad 600x, I have the wires attached (though one fell off at one point), all I am waiting for is the circuit to start working.

I read about 9.27V across the pins 4&5, so the capacitor seems to be doing its job. If anyone has a circuit diagram for the MAX232 (the insides of the chip I mean), it might be a bit easier to find the problem.

[victor]

Capacitor across #4 and #5 (C2) is working with the internal voltage inverter. Normally if you remove it, the charge pump voltage will drop significantly.

Long story short, I think you should better focus on the possible errors on your set-up.

I started to build a small GUI app to help all of you test the interfaces. I will post it here as soon as I made some tests with it, maybe tomorrow morning.

Max202 works with 0.1uF-10uF, in rest is identical with Max232. Be sure the caps are well polarized if you use elcos 1uF, specially c4 and c5, if you follow the max232 setup.

[victor]

Here is the interface tester:
www.allservice.ro/store/utils/itester.zip

There is a help window explaining how to interpret the results.

Victor

[AIM]

Thanks for the program Victor, it REALLY helped!

Having solved the problems above (connecting C5 to Vcc helped, for some reason), and also determined that one voltage rail didn't run all the way across the breadboard (my bad), it seems that my serial port, is for some reason, shot. (seems unable to produce positive voltages)

Having not used my serial port before, I guess that I won't find out why it broke (hopefully not my fault). Thanks again Victor!

[victor]

I don't think is your fault, serial ports are not so weak. If everything's set correctly, address, IRQ, etc., then I guess is a native problem with the board.
On the other hand, using a USB2COM adapter usually won't do any good, it is better to use a native UART adapter. There's a lot of PCI cards that can do the job very well and are not expensive at all.

Eventually, I hope you solved the notebook.