Serial From The Ext Port Problem

[BenScar]

Hello all,

I've put together the serial interface using the diagram from the BOB page on the WIKI (RS232 Level Shifting) but that I'm getting some very odd things out of it on boot up (see below).
Poking through the WIKI some more I found the serial cable page (Serial Cable), this shows the same circuit with a couple of differences.

From what I can see the capacitors are different values and the capacitor on pin 6 is the other way round.

Looking at the spec sheets for the MAX3232 it shows that the caps should be 0.1uF (as shown on the Serial Cable page, rather than 1uF shown on the BOB page) but in the configuration shown on the BOB page.

Has anyone else had these problems?
Would that explain the weird text I'm getting out?
Feel another trip to Maplin coming on

Edit: Got Hyperterminal set to 115200bps, 8N1, Xon/Xoff...

Any help would be appreciated.
Thanks.

Serial output at boot up:

ÿ¯ÿ÷ÿwî.¿ï9!(Nov 25 2005 - 22:51:36)                                     

U-Boot code: 03F00000 -> 03F6A61C  BSS:5ÿgp2xboot.img" from mmc 0:1 **          
1 : g_filesize 0
gp2xboot.img not found
KEÒNEÌ sum`.n.` ë » wíÛ¿ïµ½Ÿ the kernel.
÷뿽¿»2}b4voual
Memory: 30168KB available (1096K code, 784K data, 280K init)
ø 5  ¥¹ ioý ·å:mmo]W«u~t daemon v1.3.25 for /dev
øloaded
       ) ïýïõ  w{mog runlevel: 3
,c0385da0) device is not connected
gw_close> NULL qort pointez
k0385da0) device is not connected
gs_close: NULL port pointer
¬^Høßõ»¿¯¯ ¿ãâþmcq385da0) device is not connected
gs_close: NULL port pointer
«Hø{0385da0) device is not connected
gs_close: NULL port pointer
readi~g data groï õ»¿ûýËâµ½ë±;85da0) device is not connected
gs_close: NULL port pointer
«Xø7 kq;85da0) device is not connected
gs_close: NULL port pointer
vhø5 ointår
!^
MoD FLASH without ECC is not recommended

 

[MadDog]

Thats intresting, I was about to build one this weekend. Thanks for the posible heads up, i'll let you know if I hit the same prob.

 

[nickspoon]

Try using less crap terminal software. Tera Term Pro is free and works well.

 

[scorpio]

Try using less crap terminal software. Tera Term Pro is free and works well.

​

Wow! That's constructive.


To the original poster: It looks like your signal is getting smeared, most likely because the voltage levels coming from the charge pumps are not stable enough. My first step would be to use the 0.1uF caps, as per Maxim's data sheet. I had to make a phone unlocker cable a few years back using one of these, and that's exactly what I used to good effect. I should point out, however, that I was powering the driver from the PC side (from DTR, I think.)

Looking at the previous image on the wiki, it looks like it wasn't scaled right, and the decimal point perhaps got dropped off when the revised image was done.

Do you know if the 1uF caps you are using are low ESR? Perhaps the design on the wiki assumes that this is what you will have to hand. I imagine that this could have an impact, especially at 115kbps, and if they're not, then they might have difficulty keeping the charge levels constant. That said, I believe that the MAX3232 is fairly tolerant in what range of capacitors will work well with it, so if you have others lying around why not try them?

Another thing to try would be to see if you can get send a few K of text from the PC into a file on the SD card. I would suspect that data going into the GP2X should be fine, and it's when it needs to step the voltage up to reach the PC that it's having the problem. This might help define the exact nature of the problem.

Failing all that, if you could possibly get a loan of a cheap 'scope, which would probably show you exactly where the problem is with your signals. with a low-frequency signal like that which you have some degree of control over (you could put a script onto your SD card to send data through it), a normal analogue 'scope will be more than adequate.

 

[MadDog]

Yes, got mine going! yes, the caps should be 0.1uF I followed the doc in the wiki serial cable, ordered the bits from maplins, the caps I recived were the 0.1 ones so the part number is correct.

This is the first ciruit i've ever built!

For other noods here are a few links that helped me, the diagram don't match the pin out on the IC. But I think thats usual for these type of docs. One thing I found was that pin13 is real hard to workout on the gp2x connector side, so I used pin 24, worked a treat!

http://www.kpsec.freeuk.com/components/ic.htm

http://www.kpsec.freeuk.com/components/capac.htm

One thing, I can't log on, no cmd prompt. A new thread I think.

Here are the pics!

The donor parts....
http://www.maddoggames.com\gp2x\Image(53).jpg
http://www.maddoggames.com\gp2x\Image(55).jpg

serial -> usb port, my laptop has no serial.
http://www.maddoggames.com\gp2x\Image(54).jpg

The insides of the TV cabale.
http://www.maddoggames.com\gp2x\Image(56).jpg

the IC built and in its home.
http://www.maddoggames.com\gp2x\Image(13).jpg

The TV cable with its extra serial out.
http://www.maddoggames.com\gp2x\Image(10).jpg

The finished product.
http://www.maddoggames.com\gp2x\Image(29).jpg

 

[BenScar]

I thank you all, I'll be popping to Maplins tomorrow!

0.1uF caps for me. Silly mistake really, shouldn't have jumped in so fast At least it was a Breadboard rather then veroboard!

Thanks again
Ben

PS Maddog, that looks great!
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