S100 Computers

Home S-100 Boards History New Boards Software Boards For Sale
Forum Other Web Sites News Index    
SD SYSTEMS - VDB 8024 Video Display Board
This was one of SD Systems most successful products. This video board gave a rock solid 24 line X 80 characters per line B/W display. Its onboard Z80 made screen update instantaneous. It takes a parallel ASCII type keyboard for input.

SD Systems VDB 8024

The VDB utilizes a Z80 microprocessor to perform the control functions of a video terminal.  These functions include LF, CR, full cursor control, home, clear, scroll and back space.  In addition to the basic function several special additional features are provided.  One of these features provides for specified fields of characters to be enhanced.  This field can be enhanced in any of the following ways: Underline, Reverse, Blink, Protect or combinations.  Another feature of this board is the programmable character generator-  This is a 7 x 8 dot character matrix with 2 bits of descenders to provide upper and lower case characters in a 7 x 10 dot field.  128 characters are typically available while 256 characters can be made available using a 2K x 8 PROM.  These characters are displayed in a field of 80 characters in length and 24 lines per page.  The VDB processor runs only during H & V sync to eliminate any disturbance of the display due to the processor running during the raster scan.  There are several software controllable functions.  These include a speed control similar to a baud rate control and also a scroll up or scroll down.  In addition, a set of 32 special characters are available.
The VDB hardware performs all control and timing functions as well as the special enhancement functions.
The on board CRT 5027 video timer and controller is used to perform character count, character line count and row count.  The outputs of these counters are then fed into the RAM or Character Generator (CGEN) to provide the proper scanning and display of each character in its proper place.  This chip also provides for proper cursor control, horizontal sync, vertical sync and blinking signals.
The Z80 executes the software which controls all functions of the system.  This includes inputting data, storing data in memory, initializing the CRT 5027, controlling the special functions, cursor control and scrolling. 
The manual can be obtained here

An S100 board to utilize a standard PC PS2 keyboard to interface with the "ASCII" keyboard connector on this board is described here.

Some time ago Bruce Jones modified his VDB 8024, here are his comments:-

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 polarity.

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)



Other SD Systems  S-100 Boards
8024 VDB    ExpandoRAM    ExpandoROM     SBC    VersaFloppy I & II   Z8800   PROM-100  
I/O8 Serial Board  4KRAM   Other Boards


This page was last modified on 12/23/2020