After building the ASCII Video Terminal Kit, I decided to create an updated circuit board based on the original Geoff Graham ASCII Video Terminal project, which was then later updated by Peter Hizalev and sold on Tindie.

The original board by Geoff, described in the July 2014 issue of Silicon Chip magazine, included a composite video output connector, and emulated most features of the DEC VT100 and VT52 terminals. But it required the baud rate to be selected by jumpers, and had a limited menu.
The updated design by Peter did away with the composite video connector and changed the oscillator crystal from 8 MHz to a 20MHz Crystal, has a 25MHz pixel clock and 640x480 resolution at a 60Hz refresh rate. Another significant improvement is the extended setup menu, with baud rate selection and many configurable options.
While the updated design and firmware was a significant improvement over the original by Geoff, the physical board layout had many design issues in my opinion.


My board design utilizes as many of the original components from the V3 kit board as possible to retain compatibility with the existing excellent firmware. But it adds what I consider several needed features that the original design didn’t provide, such as a TTL to RS232 serial converter IC and connector, and a barrel jack for DC power. I also added a pair of single-bit noninverting bus transceivers to the TTL serial connection for voltage translation and to prevent power back-feeding from an external device.
Other component changes were:
- Changing the value of R5 from 220 to 270 Ohms for a 75 ohm impedance match on the VGA signal.
- Adding 82.5 Ohm resistors on the HSYNC and VSYNC lines to the VGA connector to reduce ringing.
- Changing the USB VBUS resistor from 4.7K to 100 Ohms.
- Changed the 3.3 volt regulator output capacitor from 10 uF to 47 uF.
- Additional bypass capacitors on U1 (PIC32) and the 3.3 volt regulator.
- Added a 250 mA Polyfuse and 470 uH inductor on the USB power VBUS.
- Changed the pullup resistors for data and clock on the PS2 keyboard connector to 5.6K Ohms.

The revision 1.4 board layout has changed considerably from the original version 3 board by Peter. Some of this was necessary due to the addition of new components, but mostly due to optimizing component placement and the circuit traces between them.
The first thing I wanted to improve upon was the power traces to all components, and to include a ground pour instead of the original daisy-chain point to point ground of the original V3 layout. The next improvement was reducing the length and increasing the isolation of the video traces from other digital and power traces.
I also did my best to impedance match the USB data traces, which was likely not necessary due to the short connection distance.
Bypass capacitors were placed as close to the power connections of devices as possible to reduce inductance. Also the crystal was placed next to the microprocessors OSC pins using wide traces to reduce inductance, and isolated from other traces with ground pours.
The board is now slightly larger and measures 2″ x 4″ (5.08 x 10.1cm) which is a half inch wider than the original V3 board.

The updated revision 1.4 boards worked very well, with less pixel smearing on the display and a power consumption of 45 mA at 5 volts without the PS2 keyboard attached.
The only issue that I ran into was when trying to reorder the 74LVC1T45 buffer IC’s. No one had them in stock, and lead times were over 9 months long.
That drove my decision to design another revision of the board using a Maxim MAX12931EASA+ 2‑channel, digital galvanic isolator. The MAX12931 is available in a SOIC‑8 package which makes it a little easier to solder for a surface mount package and only requires one IC. When I redesigned the board, Mouser Electronics had quite a few in stock so I ordered 10 of the IC’s.
As I am writing this post most of the suppliers are out of stock of this part also.

If the board will be used solely for RS-232 serial communications, then the TTL buffer IC is a non-issue. I am still waiting on my Rev. 1.8 boards, and there are only minor changes with trace width and swapping the TTL buffer IC’s so I am not expecting any issues with the new boards.
EagleCAD 7.7 schematic and board ZIP files for Revision 1.4
BOM for Revision 1.4 board
OSH-Park project page for ASCII VT100 Terminal Rev. 1.4
EagleCAD 7.7 schematic and board ZIP files for Revision 1.8
Looks like a worthwhile update. Do you have any spare (bare) boards? Don’t need five of them, so prefer not to order another set from OSHPark if possible. I will only be using RS232 input, so indifferent to buffer style.
Thanks!
Hi Jack,
Replied to your email address.
Greg (Barbouri)
No one uses eagle cad anymore. (except you)… LOL
That’s a pretty broad and definitive statement. I’m pretty sure that there is at least one other person still using Eagle Cad.
I actually paid for my version of the software, and have over 600 designs in Eagle Cad format.
I have tried in the past to convert some of my designs over to KiCad with very poor success. KiCad is getting better and better, but the import function is severely lacking.
So far Eagle Cad meets all my needs and the files can easily be converted over to Gerbers, so I will stick with it for now unless someone want’s to donate a copy of Autodesk Fusion.
Greg (Barbouri)