SDS VDB-8024 S-100 Card

In Sept 2003 there was a discussion in newsgroup comp.os.cpm on the SD Systems video card model VDB-8024. Bruce Jones described modifications to the card which converted it from NTSC (TV-type) video to VGA. With his permission, the mods are described below. Mr. Jones provided code for the card: here is a link to download it. If you wish to contact Mr. Jones, contact me (Herb Johnson) and I will forward your inquiry. For additional SD Systems information and software, check this link to my SD page.

To return to my S-100 home page, follow this link. Material on this page (c) copyright Herb Johnson 2003, except for material provided by Bruce Jones with his permission, for which he retains copyright.

From: "Bruce Jones"
Subject: RE: SD Systems VDB - 8024 Conversion for VGA Monitor
Date: Tue, 30 Sep 2003 09:39:04 -0400

Hello Herb,

All is working well with the VDB-8024 modification, so here it is.



To allow the S.D. Systems VDB-8024 CRT board drive a PC VGA Monitor. You
will have either white or green characters on a black background, depending
upon your preference. The display area will exactly fill the entire CRT tube
area, though you may have to adjust for vertical and horizontal height and
width in some cases. Cost of the modification is about $5.00 to $7.00.


1) 1 28.63636 Mhz crystal for Y1(or as close as you can get to this frequency)
2) 1 74ALS163 (or 74H163 or 74S163 or 74F163)for U1
3) 1 74ALS165 (or 74H165 or 74S165 or 74F165)for U24
4) 1 74ALS04  (or 74H04 or 74S04 or 74F04) for U2 ** ONLY IF U2 is a 74LS04 **

Once the mods have been made, you can switch back to the original operation
by replacing the original 14.43 Mhz crystal at Y1 for the new 28.63636
crystal. The new ICs stay in place. You can even leave the trace changes
under U1, though the onboard Z80 will run slower.

Mods Required for SD Systems VDB - 8024

a) Y1 - change from 14.43 Mhz to 28.63636 Mhz - basic timing element

b) U2 - if a 74LS04 is original, change to 74ALS04 for oscillator circuit

c) C3 - lift one lead from trace to "remove" (may not be necessary, but I did it)

d) U1 - change from 74163 to 74ALS163 for character clock and Z80 clock
generation. Also lift pin 13 on U1, and short trace pads U1-12 to U1-13,
giving a 2.5 Mhz clock again.

e) U24 - change from 74LS165 to 74ALS165 for video shift register.

The CRT5037 now generates a horizontal signal of about 31Khz, and a vertical
sync pulse of 120 Hz. This seems not to bother the VGA monitors. I have run
it successfully on a low end 15" VGA and mid quality 17" VGA monitor with
perfect results.

CRT Cable to VGA Monitor Wiring

+Horizontal drive to pin 13 on VGA connecter
-Vertical drive to pin 14 on VGA connecter

For White Characters on Black Background

+video signal out to pins 1,2 & 3 on VGA connecter (RGB)

For Green Characters on Black Background

+video signal out to pin 2 VGA connecter (Green input)

Ground to pins 5,10 typical on VGA connecter.
(and 6,7 & 8 if needed for RGB return, but this was not done)

While H and V polarity are shown as +H and -V, most VGA monitors sync on any


The onboard video memory, U3 to U6 (2114 1K X 4 RAM) will now be accessed at
double the previous speed. On a cleared screen (all 20H charaters) you may
see a few odd characters. If so you will have to change one or more of the
video RAMs to a higher speed part. BG Micro has them for $1.15 each.


While the new clock signals are within the specs of the CRT controller, it
will run a bit warmer. If you have a warm interior to your system, you could
fasten an old '486 heat sink to the CRT5037 by passing single strand #18
gauge wire under the IC, back up through the cooling fins and twisting the
wire to tighten the grip. Be careful not to stress the IC package. Use
thermal transfer paste under the heat sink.


Installing a 28.63636 Mhz Oscillator in place of a crystal (with even more
mods needed)
Running the onboard Z80 at 5Mhz (without the U1 trace change, that will be
the speed).


*>Some VGA monitors do not display well with just a Green input signal active.
*>However, in all cases to date, connecting the single video output from the
*>VDB-8024 to the RGB pins, which gives white characters,all tested monitors
*>have performed perfectly. To do this connect the VDB-8024 video signal to
*>pins 1,2 & 3 for the VGA cable. (rcvd 14 Oct 03)

In Nov 2003 Bruce contacted me and provided some of his background with SD Systems, and a possible upgrade for the VDB software. COntact me (Herb Johnson) for availability.

Hello Herb,

Aside from getting the VDB-8024 driving a VGA monitor, I also improved the
firmware back in 1981. One of the outside programmers began using Wordstar
for coding on the Zeus80 system I developed. He mentioned that getting line
insert/delete functionality would speed it up by about 10 times on the
screen, each time a person inserted/removed text. I thus added that
functionality, and it works very well, making Wordstar much less clumsy at
many screen updates. I also added EOS or EOL, or both. One or both of those
were missing in the original VDB-8024. If you would like the source, HEX and
BIN files for this let me know. I also have a version which emulates a
subset of the Televideo 920 but I should test it out again before letting it

To be sure there is an improvement, I re-installed Wordstar for an ADM 3A,
which uses the same cursor-positioning codes as the VDB-8024, but has no
line insert/delete. I was nearly ill waiting for screen updates, so yes, it
really helps.

Best regards,
Bruce Jones

Herb Johnson
New Jersey, USA

Copyright 2003 Herb Johnson
Last updated Dec 31 2003