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PC To Calculator Link

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evilgecko

Sat Apr 22 2006, 11:38PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

I built a little circuit which allows you to connect a Casio calculator to computer using a serial port and the stereo 2.5mm phono plug on the calculator. The circuit schematic I got from PMB Electronics: Hobbyists Corner - Casio Graphic Calculators, I deisgned a PCB using eagle.

I finished it yesterday and tried it out. Using the Casio F-123 program deisgned specfically for transfering data between your graphic calculator and PC, I was able to send data to the calculator but the computer was unable to recieve it (the error was "Transmission failed!").

This one way street is annoying. I've had a look over the board but can find no joining tracks or whatever. I think I should scope it out but don't know what to look for.

1145749132 288 FT0 Pc To Calculator Overview Low Quality

1145749132 288 FT0 Pc To Calculator Pcb Bottom Low Quality

evilgecko

Sat Apr 22 2006, 11:43PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

For some reason the schematic and PCB board layout didn't upload, so here they are.

1145749422 288 FT8249 Pc To Calculator Schematic

1145749422 288 FT8249 Pc To Calculator Pcb Design

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Sun Apr 23 2006, 12:09AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

arrg, whoever made the pictures open in the same window when you are composing a post should die... I was like 90% done then I click the schematic and I loose the post

In any case...

I would resolder the 5th pin down on the left (it seems a little shakey) and the heat up the 5th pin on the right and the other pin it is connected to.

If that doesn't help...

folow these simple steps to find the problem...
1. scope the pin that the calculator is supposed to be transmitting on. If you see 5v pulses when the calc is transmitting (don't worry about the time base on the scope, just look for the trace to jump from 0v to 5v like crazy) then go to step 2. If you don't then chek the wireing to the calc.
2. scope the pin that connects to the serial port (the one that transmits to the comp). If you see +/-12v pulses here then check you connector pinout. If you see small pulses then go to step 3. If you see nothing then go to step 4.
3. Scope the 5v power to the max232 chip. It should stay at 5v all of the time. If it doesn't then you could try adding a decent sized electrlytic across +5v/gnd, or power from a floating 5v supply. Then scope pin 2/6 and make sure you have a steady +/- 12v on them.
4. Check the pinout the max chip, and check for shorts.

good luck!

evilgecko

Sun Apr 23 2006, 01:57AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Hey thanks I'll try your problem solving guide soon. Is it ok to test the connectivity between two tracks with a multimeter? Just to make sure they are not connected. Or would the small voltage from the DMM kill the chip with undue currents?

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Sun Apr 23 2006, 04:20AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I wouldn't worry about it... But some pins will probably show up as shorted inside the chip...

In the future it would be a good idea to socket the chip so that if you burn it out you can easly replace it, and you can pull it out to check for problems.

Bjørn

Sun Apr 23 2006, 12:47PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Some of the solder joints does not look optimal and may need to be redone. The solder blob should be concave, if not there is something wrong with it.

evilgecko

Sun Apr 23 2006, 11:18PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Yeah sorry about all the solder mess, I was using a fat chisel tip as the local electronics store has stopped selling solder tips, and the copper was starting to corrode making it very difficult, even when sprayed with a laquer. Is there any way to stop the copper losing its shine?

Anyway I fixed it up and now it works both ways. The calculater lies and say transimission failed still though. I'm planning on making a few more for other people and I think I can change the large LM7805 5V regulator into its little 100mA brother. That wouldn't cause any heating issues would it?

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Mon Apr 24 2006, 12:16AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Good to hear it is working

The 100ma one should be fine.

rupidust

Mon Apr 24 2006, 01:38AM

Banned
Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85

--- is right, but why?

You first measure your circuits current. Say Icircuit = 20mA, now compute how much power a 5v regulator will dissipate. (Vin - Vreg) * Icircuit. Next measure the voltage at PC's DTR line when asserted + and loaded. Max is 25v but it most likely will be no more than +12v, +9v these days. Recompute: (12v - 5v) * 20ma = 140 mWatts. 140mW is within the 1 to 2 watt range of a LM78L05 TO-92 package. Basically if the input voltage is under the Max from the datasheet and the regulators current for the circuit is under the Max from the datasheet, it will work.

evilgecko

Mon Apr 24 2006, 07:54AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Rupidust, thanks I didn't realise that a voltage regulator was limited by the wattage as well as current, but I understand now why it should be.

How do I keep the copper shiny?

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