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ColecoVision diagnostics shield

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#1 ChildOfCv ONLINE  



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Posted Sun Mar 10, 2019 6:52 PM

Maybe more for idle fun, but also to aid in diagnosis, I am working on an Arduino shield for the Mega 2560 that connects to the expansion port on the ColecoVision, in order to diagnose as many hardware problems as it practically can.  I'm only in the beginning stages at the moment.  First check is clock functionality (easy enough), then that the bus behaves like it should (check that the processor disengages when told, and that none of the bus lines are shorted).  Planned additions will be ROM checksum, RAM and VRAM checks, cartridge ROM check (though this would require a 32K cartridge to fully check all access lines), sound and VDC checks, and possibly even checking the RF and video switcher directly.


The shield board is not technically advanced--basically it just maps most of the expansion port pins to Arduino pins, and the diagnostic software will do all the testing itself.


I did add a rudimentary DAC that I mostly cribbed from the Atari for video output, though it will only truly be useful for black/white images at best.  Still, if I enabled the external video and sent a "composite" signal through it, the RF modulator should be able to make it appear on the screen.


Anyway, just ordered a prototype PCB for this.  We'll see how long it takes to arrive.


Screen Shot 2019-03-10 at 12.15.32 AM.png


If interested, I've added the board design to my CV schematics under the Diag sub-project.



#2 omf OFFLINE  



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Posted Mon Mar 11, 2019 5:16 PM

Don't suppose you have any schematics or other useful information for the French SECAM model colecovision?


Information on that is rather rare unfortunately.  i have been trying to fix the video output on mine but i have not found the worn out component as yet.

the machine works but after a short time i get white ghosting on the screen so must be a old on its last legs component.


i have changed all the chips apart from the z80,

i have also changed the video transistors

still the ghosting is present.



the arduino shield idea looks like an interesting project and ill be following.

#3 ChildOfCv ONLINE  



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Posted Mon Mar 11, 2019 9:09 PM

Unfortunately I don't have a SECAM version to play with.  I guess I could see if one ever gets listed on eBay for cheap and then trace it out.


So that version doesn't seem to have an RF connector?  I guess you just connect it to the SCART input?


Well, one thing that might help for reverse-engineering it would be if you could take some high-res photos of both the front and back in the upper-left area.  Like, straight-on view with a good DSL camera from a distance with a good lens to minimize parallax error.  And of course in good lighting so that nothing is hard to see.

Edited by ChildOfCv, Mon Mar 11, 2019 9:09 PM.

#4 ChildOfCv ONLINE  



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Posted Thu Mar 14, 2019 1:57 AM

So far so good... 

Choose a test mode:
1)  Basic functionality
CLK0: 126:123
CLK1: 130:119
CLK0 seems ok
CLK1 seems ok
RESET recovery time: 591
Checking BUSACK
Checking bus lines
Bus test pass
Checking resting state
Resting state looks okay
Checking wait state
WAIT: 79:170
WAIT seems ok
Checking U5
U5 pass

I think next check will be CPU address space (ROM checksum, SRAM check, cartridge ROM check).

#5 ChildOfCv ONLINE  



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Posted Fri Mar 15, 2019 12:42 AM

ROM checksum done...at least for American unmodified ROM.  I guess I'll need to find all the different variations out there and build a database of CRC32 checksums if I want this to be more than a regional thing.


And my latest facepalm is finding out that I hooked up the data lines in the reverse order.


Anyway, next is the RAM test.

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