
I decided to build an ASCII VT100 video terminal to go with my Tauntek logic IC tester.
I came across “Geoff’s Projects” website where he describes an ASCII VT100 compatible video terminal that he built back in 2014. Since then several people have published updated versions of the video terminal with different features. I decided to check out an updated kit by Peter Hizalev.
The complete kit is available on Tindie for $52.00 USD plus shipping, which sounded like a reasonable amount for a complete kit with a pre-programmed PIC microcontroller.


The kit consisted of a nicely wrapped PCB, and several small plastic bags of components, grouped by connectors, passive components, IC’s, etc.
This was my first chance to get an up-close look at the PCB, and to decide in what order I would be installing the components.
The first thing that stood out to me was the plated mounting slots for the PS2 keyboard connector, and I said to myself “nice job on the slots”. But then I continued to look at the board’s trace routing with disappointment. There are so many things wrong with the boards component placement and routing, that I would have to say this is the worst routed kit circuit board I have ever seen.
I remember some of my very first circuit boards that I designed, and they weren’t much better than this one. But they were for my own projects and learning experience, and I didn’t sell them as a finished product.

I am not a big fan of CAD autorouters, but sometimes they are a great start for a complicated PCB.
This is not a complicated PCB.
If using an autorouter it is usually prudent to visually check the finished routing and make some adjustments in component placement, or hand route oddly placed connections. It seems none of that took place on this board.
All the traces are the same size (10 mil). Typically power and ground traces are larger than small current / signal traces. The power and ground traces are daisy chained from component to component on this board, with very thin conductors which is not good practice.
The 20 MHz crystal oscillator and capacitors are mounted more than halfway across the board from their microprocessor terminals, and are routed next to video and other traces. The crystal and associated components should be mounted as close to the microprocessor’s OSC pins as possible using thick traces. This helps reduce parasitic inductance & resistance. Typically a ground plane should be provided under the crystal and tied directly to the microprocessor ground, or at least a guard ring.
Another issue was incorrect placement of bypass capacitors for the microprocessor. The PIC microprocessor datasheet specifies the requirements and recommendations for bypass capacitor size and placement on the PCB. The 0.1 uF capacitors were placed so far away on the PCB I thought they were for something other than the microprocessor. Bypass capacitors should have wide traces to reduce inductance, and be placed as close to the power and ground pins for the component they are bypassing as possible.
And the list of issues with the physical circuit board goes on and on.

Much to my surprise, after testing and installing all of the components, the terminal board actually worked.
The setup menu screen for the terminal settings works extremely well, and I was able to quickly set it up for use with my Tauntek logic IC tester using an older VGA monitor.
The menu supports Baud rates from 110 to 921,600, with one or two stop bits, and parity of none, even, or odd. The “Terminal” section of the menu has 14 selections for items such as selecting the character set, keyboard compatibility, XON/XOFF flow control, and many other functions. The menu was easy to navigate, and stored the setting for the next power-up.

Even though I now have a functioning ASCII video terminal, I cannot recommend purchasing this version of the kit.
I did end up changing the value of R5 from a 220 Ohm resistor to a 270 Ohm resistor. This is the resistor for the analog R/G/B 75 Ohm video signal and should output 0 to 0.7 volts.
I also ended up changing out R10 and the blindingly bright red LED, to a diffused green LED and a much higher value resistor to reduce the LED current by 50%. The LED is always lit when the board is powered on, and flickers off and back on during serial data transmission.

After initially writing this post and posting a review on Tindie, Peter responded and thanked me for an honest assessment. Also he didn’t use KiCAD autoroute for the trace routing as I mistakenly assumed, and routed the board manually.
Also Kicad autorouter was removed in version 5.
It seems that he is genuinely interested in improving his design, and had several questions on how to learn more about board design. It was refreshing to have a constructive discussion with someone who is willing to improve their skills and their project.
He is working on improving the design. So I am looking forward to see the next version of the VT100 ASCII Terminal.