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CParish
January 3rd, 2016, 10:31 AM
I've been restoring/upgrading an 8088 Compaq Portable and I was wondering if it is possible to connect a normal EGA card to the internal CRT? The service guide for the Portable 2 lists the existence of two 8-bit "Enhanced Color Graphics Boards" with the requisite internal connector, but I don't have those.

I determined the pinout of the connector that runs from the MDA/CGA card to the CRT, and it should be relatively straightforward to connect an EGA card with some op-amps and timing adjustments to the CRT. Just to try things out, I swapped in a Zenith Z-449 EGA/VGA card and built a quick board for experimenting with the primary green channel. The CRT did sync at mode 16 (21.85kHz) and drew mostly correct, so I think it's possible. Has anyone here tried this before?

For reference, the pinout of the VDU->CRT connector on my system is:

----------------
| 1 2 3 4 || 5 |
| 6 7 8 9 || 10|
----------------

1 - Red - VSync - Negative Pulse ~150us All Modes, 50Hz MDA, 60Hz CGA
2 - Black - CGA Mode Active? - High in CGA mode, low in MDA mode. My CRT appears to multi-sync and ignore this.
3 - Orange - Analog video data - ~3V max observed
4 - Brown - Hsync - Positive Pulse ~5us All Modes, 15.7kHz CGA, 18.5kHz MDA
5 - Grounding Strap
6,7,8,9 - White - Video Ground
10 - NC

PhilipA
January 23rd, 2016, 06:42 AM
Haven't tried but was thinking about it.

I need to recap the VDU board on mine because it gets a bit wobbly and the blanking circuit isn't as wonderful as it should be.

EGA Wonder had an addon, as you say that likely just took the luminance average of the 3 color channels- though wiring up as you have in the same fashion that a mono EGA monitor would work.. can't see why not, it's within the ball park of MDA/CGA for the screen to work.

Does yours click between sync speeds if you go to 40/80 line modes? I'm thinking that the signal on the black will trigger that change.

--Phil

CParish
January 24th, 2016, 04:13 PM
It looks one of my CRT boards is of Zenith origin. It doesn't have a relay that selects between the sync speeds like the other ones I've seen, instead it locks onto the horizontal sync rate with what looks like a PLL, although I didn't look too hard at it. The back porch for EGA signals is too small by over a micro-second, so the image is shifted significantly to the left. 1us is quite a bit too long for analog delay lines, although it is possible to build a sync circuit that generates the pulse early (so the image is centered)

In the intervening time, I did manage to pick up an EGA wonder (without the unobtainium Compaq interface module). When switched to Compaq mode, it outputs video on the feature connector at a fixed 18.432 kHz, the MDA horizontal rate. My guess is that they didn't want to run things out of spec (or maybe the relay carrying Compaq CRT boards won't lock at higher frequencies). Either way, I'll build a board that plugs into the feature connector and sums up the various color/intensity bits into something analog-ish for the CRT to consume.

PhilipA
January 25th, 2016, 10:31 AM
So, by the sounds of it, the Compaq mode is color MDA. Or, well, grayscale MDA compatible in terms of timing?

Phil

vwestlife
January 26th, 2016, 06:16 AM
Compaq did release an updated video card that supported 640x350 EGA graphics on the Portable's built-in monitor, and some aftermarket EGA cards supported the Portable's CRT as well. This web page references it:

"Apparently EGA cards existed with the appropriate connector for the Portable's internal monitor. I've seen references to the Compaq Enhanced Color Graphics cards (in versions I and II) and to an add-on module for the ATI EGA Wonder."

http://www.seasip.info/VintagePC/compaq.html

CParish
January 30th, 2016, 10:35 AM
So, by the sounds of it, the Compaq mode is color MDA. Or, well, grayscale MDA compatible in terms of timing?
Phil

Yep. EGA wonder always runs with MDA timing in compaq mode (that I've seen). I threw something together to take the various color TTL lines off the feature connector and make the analog signal for the compaq CRT:
29319

It works pretty well, although I dont have the color shades adjusted quite right yet. The 5V TTL video bit times are pretty short, so I chose the AD8055 op amp for its high gain bandwidth, slew rate, and unity gain stability: http://www.analog.com/en/products/amplifiers/operational-amplifiers/high-speed-amplifiers-bandwidth-greaterthanequalto-50mhz/ad8055.html#product-overview
29320

PhilipA
January 30th, 2016, 10:48 AM
That looks good!

I am still kicking myself for selling my old EGA Wonder. Oh well. Live and learn. I think I'll see about picking one up. EGA is a nice thing to have, over CGA.

--Phil

soviet9922
September 12th, 2018, 11:37 AM
Hi can you post a schematic or a diagram of this ?

Casey
September 13th, 2018, 06:41 PM
Impressive!