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XT/CGA laptop dissection

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First post, by bjt

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Unearthed this from the garage where it's been sitting for around 15 years.
Thankfully the battery was removed so no leakage. However, it wouldn't POST so I thought I'd take it apart to see what's up.
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This one would have been fully loaded in 1989ish. 1MB ram, 10Mhz 80C86, 720kB floppy, 20MB hard drive.
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Never seen a JVC hard drive before! Unfortunately it's shot, spins up but then shuts down again. After reseating the drive controller PCB it would post but gave a 1701 error. I've removed the HDD and controller and plan to run it off floppy and over Interlnk.
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It also wouldn't boot off a floppy at first but cleaning the drive heads with some alcohol helped. It's alive!
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The LCD has 4 shades of grey and has terrible ghosting so no good for games.
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What's next?

- Replace the RTC battery as it's losing time
- Give the case & keyboard a good clean
- Build a CGA-Scart cable and get it hooked up to a TV

Reply 2 of 100, by bjt

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Any ideas how I can make use of the 384k upper memory? The BIOS setup has an option to enable expanded memory, but I presume I would need a DOS driver for this too? Any ideas what driver this might need? Looks like the chipset is made by Chips & Technologies.

Reply 3 of 100, by idspispopd

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I suppose you will need to know which C&T chips are used exactly and research from there. According to the Wikipedia page on Chips and Technologies it could/should be 82C100. "The Last Byte Memory Manager" documentation mentions this chip.
http://www.google.com/patents/US5163153 has some information about this chip and EMS. RBINT also mentions the EMS control register.
Couldn't find a driver, though. The drivers on http://ibm-pc.org/drivers/memory/memory.htm seem to be meant for later C&T chipsets but you might want to try these.

Reply 4 of 100, by bjt

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Thanks for looking into this. Yes, it has the 82C100 on the left had side of the board pic above.

I think I'll try the EMM.SYS for the NEAT chipset, as they are meant to be closely related. I'll also look at the Last Byte manager.

Reply 5 of 100, by keropi

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^try the demo of TLB and report back if it works , my bet is on that one - though I never knew that TLB can be used to add any memory other than UMB blocks...

cT89QGt.gif Amstrad PC7486SLC-33 wanted , click me for AmiBay ad!

Reply 6 of 100, by bjt

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Replaced the CMOS battery, but my soldering skills aren't pretty enough to show here 😒 It keeps time now which is cool.

Interestingly the CMOS battery wasn't a standard battery after all. It's actually a supercapacitor - a special capacitor with higher energy density. It's a shame more old boards don't use these as they don't have a volatile electrolyte to leak out and cause problems. http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

Also pulled an old 14" TV with SCART out of the garage to serve as a monitor. I'll be using the "Pull Both V2" circuit here: https://sites.google.com/site/h2obsession/CBM … 8/rgbi-to-scart

Looking forward to playing the old Apogee CGA games, ZZT etc. on this. The crappy CGA text font & scanlines are a different experience to VGA.

Reply 7 of 100, by bjt

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Well, the NEAT driver didn't work, and although the Last Byte memory manager detects the 82C100 chipset, it fails to initialise with the error
"High Ram Missing/Defective/Disabled/Relocated!"

I think my best bet is to use the original C&T EMS driver, but so far I've not found it on the net.

Reply 9 of 100, by bjt

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Some progress. Built the SCART and CGA cables. Built the enclosure and wired up the PS/2->XT keyboard adaptor. I'll be taking 5V from the keyboard to power a quad XOR IC to generate composite sync, and to provide the RGB blanking signal to the TV.

Just need to build the actual circuit for RGBI->RGB and composite sync generation.
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Reply 10 of 100, by bjt

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Built the CGA->Scart circuit, works fine on my LCD TV

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However unfortunately it doesn't work fully on the CRT TV I was planning to use. I'm using this circuit: http://www.vintage-computer.com/vcforum/showt … 3710#post333710.

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The image is more or less OK horizontally but rolls slowly vertically. Maybe this TV doesn't like 60Hz but I'm sure I used to play NTSC PlayStation games on it. These would have had sync as part of composite video signal though. I suspect perhaps the XOR gate can't provide enough current to the sync signal, I might try bumping the sync voltage from 0.3 to 1V or add a transistor driver stage.

I have also ordered a Covox clone from here: http://retrocables.es/tienda/index.php?id_pro … oduct&id_lang=1
The floppy drive is still a bit flakey so I'm going to replace it with the HxC slim I bought recently.

Reply 11 of 100, by Mau1wurf1977

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CGA to SCART! Wow I had no idea something like that existed 😀

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 13 of 100, by Mau1wurf1977

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I must say the picture on the LCD looks really crisp. Shame about the CRT...

My website with reviews, demos, drivers, tutorials and more...
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Reply 15 of 100, by ripsaw8080

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The palette 0 brown seems pretty far off on the LCD but looks fine on the CRT...

In the CGA->SCART schematic, I'm curious about why intensity (6) is connected to the red channel (3) through the network of three parallel resistors.

Reply 16 of 100, by bjt

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Yep, this doesn't have the dark yellow->brown fix. It's possible but makes the circuit much more complex.
That's an error in the schematic, each of the three intensity lines should be connected to one of the colour channels.
Also, it's not necessary to use the first two XOR gates (IC1A/1B) because CGA sync polarity is positive.

Reply 17 of 100, by bjt

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I added a transistor output buffer to the sync signal to rule out any lack of current issues and found that bumping the sync voltage to ~1.9V fixes the 40 col/320x200 modes. The 80 col/640x200 modes still roll vertically on the CRT but are fine on the LCD TV. This is weird because the only difference between 640 and 320 modes is the frequency at which the colour lines change. Any further suggestions welcome as I'm at the limit of my electronics knowledge 😎

Also added a headphone socket so that I can get Covox sound out of the TV.

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Reply 18 of 100, by bjt

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Next up - get the HxC slim floppy emulator working. The floppy pinout on this machine is nearly PC standard as far as I can tell, with a couple of important differences:

- +5V on pin 1 (normally this is ground)
- Disk change (pin 2) and ready (pin 34) are flipped compared to PC

The stock drive is a Chinon FX-354 which was also used in some Amigas and keyboards. There's no separate power connector so it gets +5V through the floppy cable. Using pin 1 is a bit nasty because if a standard floppy drive is plugged in it'll short 5V to ground 😵

I need to build a short cable to connect to the HxC anyway so I can break out +5V and ground to a floppy power connector. One cool feature of the HxC is that it can emulate both A: and B: at the same time with different images loaded in each. Since the HDD is dead I'm planning to use this to have the OS files on A: and switch disks in and out on B:. This will require porting the HxC selector software (http://www.youtube.com/watch?v=9R36HT_S0Cg) to PC.

Reply 19 of 100, by cdoublejj

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I wonder if my brother's laptop has a video out or if one can be made modded,i also wonder if the screen can be upgraded/modded/replaced. it's a win 95 era laptop. I think it's an Epson.