ASCII Video Terminal

Geoff Graham released an inexpensive video terminal design in 2014. Several others have updated the firmware. Here is an updated PC board.

Description

Geoff Graham released an inexpensive video terminal design in 2014.
Unfortunately the original design only had TTL serial output and it was necessary to add external drivers for RS232 levels.
Several others have updated the firmware including allowing use of a USB keyboard.

Here is an updated PC board that incorporates several changes:
RS232 driver is included (Max232CPE or equivalent)
Color selection is now thru jumpers.
USB connector changed to a USB "A" connector so a keyboard can be directly connected
Barrel Jack input for 5v added
Silk screening improved to show jumper settings.

The original firmware will support a PS/2 keyboard input
David Hansel's modified firmware will support a USB keyboard.

I've built several terminal boards over the years. While the font isn't as nice as some others, this simple terminal board has many advantages. It uses easily available thru-hole parts, the source code is freely available, and it supports a wide range of baud settings.

The credit clearly goes to others for the design. I wanted to learn KiCad so used this as an opportunity to complete a simple project.

Please enjoy!

I have uploaded the gerber files to PCBWay to make ordering easy. Current cost for 5 boards with shipping to the USA via DHL is $22:

https://www.pcbway.com/project/shareproject/ASCII_Video_Terminal_2.html

V1.0 of the PC board had the composite video connections backwards. If you use a connector off the board and correct the wires it will work fine.

V1.1 and V1.0 did not account for the PIC32 not liking TTL voltage levels. You can modify the circuit with a 220K resistor and trace cut as documented below.

V2.0 changes the RS232 driver to a MAX3232 which will output the correct 3.3v levels for the PIC32. No trace cuts or modifications are required. Also Capacitors C8-C12 can be either 1.0uf or 0.1uf (they are 1.0uf on the PC board and BOM).

Files

BOM (revised for V2.0).txt

Bill of materials (revised for V2.0)

plain - 942.00 bytes - 10/23/2020 at 18:06

Download

Schematic.pdf

Schematic (updated for V2.0)

Adobe Portable Document Format - 59.27 kB - 10/23/2020 at 17:59

Preview

Graham Video Terminal USB Keyboard V2.0.zip

Gerber files for fabrication V2.0

x-zip-compressed - 43.85 kB - 10/23/2020 at 17:57

Download

Graham Video Terminal.step

.STEP file

step - 16.93 MB - 10/05/2020 at 22:41

Download

Graham Video Terminal.jpg

3D Image of PCB

JPEG Image - 103.83 kB - 10/05/2020 at 22:40

Preview

Discussions

Mike Fetting wrote 10/07/2020 at 03:16

Ordered v1.1 boards, and from the comments I understand I need to cut the trace to pin 5 and add a 220k resistor. Anything other mods?

I will work on a 3D case once I have the board assembled.

Are you sure? yes | no

Ken wrote 09/24/2020 at 15:40

I've ordered up some of the Rev 1.1 boards from PCBWay and am looking for footprint-compatible VGA, PS/2, and serial port connectors. Which ones in particular were used in this design?

Are you sure? yes | no

gkaufman wrote 10/01/2020 at 01:05

Ken -

I picked up the connectors a long time ago thru Aliexpress (back when Chinese shipping was quick and cheap). All are very standard connectors and available thru Mouser or Digikey.

Are you sure? yes | no

williad wrote 09/13/2020 at 10:17

Hi Gary,

I have looked through the Max232 & Pic data sheets. It looks to me that 1k0 would be the smallest value of series reistor to use between MAX232 pin 12 & PIC pin 5, but to allow for tolerances I would suggest trying a 1k5 and then measuring the voltage on the PIC pin to see if it has dropped to around 3v0 . If this resistor is made too big then it will limit the maximum baud rate on receive. I haven't built my board yet so can't check the voltage myself.

Are you sure? yes | no

gkaufman wrote 09/18/2020 at 16:07

To get the voltage at PIC pin 5 to drop to 3.5v took 220K between the MAX232 and PIC. Good news is that it still works fine at 115.2K baud. I don't know if it pays to use a voltage divider instead of a simple resistor? Geoff Graham used 100K to 3.3v and 10K in series.

Are you sure? yes | no

gkaufman wrote 10/01/2020 at 01:03

A single trace cut as pictured above, with a 220K resister added dropped the voltage at PIC32 pin 5 to 3.5v on my board, and seems to work nicely.

I have re-done the PCB to use MAX3232 IC powered by the 3V3 rail. They have been shipped to me and I'll test them when they arrive.

Hopefully this will be the last revision!

Are you sure? yes | no

williad wrote 09/07/2020 at 11:00

Hi Gary,

Thank you re comments on the Composite RCA socket & for redesigning the PCB.

However, while checking resistor values I have noticed a subtle error. Pin 4 & 5 of the PIC are NOT 5V tolerant. Their maximum permitted input in Vdd + 0.3 V ie about 3V6. If you look at Geoff Graham's schematic you will see a 10k resistor in the RxD line between CON1 & pin 5 on the PIC. Geoff says this resistor is there in case a 5V TTL adaptor is plugged into CON1 to protect the PIC pin 5. The subtle fault I noticed is that RxD (pin12) of the MAX232 is connected DIRECTLY to the PIC pin 5. The MAX232 will output 5V on pin 12. There should be a 10k resistor fitted in this line to protect the PIC pin 5. As the MAX232 IC is fitted I don't think you will be able to use CON1 (unless you pull out the MAX232 IC). Thus you could cut the PCB track from pin 12 of the MAX to pin 5 of the PIC & connect it to CON1 pin 2 & so use the existing 10k resistor.

Another possible solution would be to use the MAX3232 IC powered by the 3V3 rail, as this chip would only put out 3V3 on pin 12. C11 in this case would need to go to 3V3 not 5V.

Are you sure? yes | no

gkaufman wrote 09/10/2020 at 13:00

ARGH, always one more bug. Thank you for being so observant!

I'll try and edit the pcb this weekend and get a revised board fabricated.

I'm measuring 3.9v on RxD (PIC pin 5), so indeed over the 3.6v max. It does appear to work fine, but needs fixing!

I assume that putting a 10K resistor between PIC pin 5 and MAX232 pin 12 would do the trick?

I did leave the CON1 option in place, but realize that to use it you'd need to remove the MAX232.

Pretty easy to cut the trace on the V1.1 board and solder a 10K resistor on the back of the board. I'll get some pictures of the changes up later this weekend.

Are you sure? yes | no

s_brune wrote 09/25/2020 at 23:52

Have you decided on how the design will change for this fix? Is it a single resistor or a voltage divider? What value(s) are needed?

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s_brune wrote 09/06/2020 at 19:42

Is there a recommended enclosure to package this project?

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gkaufman wrote 09/06/2020 at 22:47

I wish someone would design a 3D printable case, but I don't have the expertise. If anyone is interested in tackling a case please contact me. I'll gladly provide a working board.

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jutzul wrote 10/21/2020 at 18:29

I do not have the PCB yet, but I designed in F360 a preliminary enclosure using the .STEP file generated by KiCAD. Feel free to download and modify it:

https://a360.co/3m9kuES

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gkaufman wrote 09/04/2020 at 15:12

I have constructed the corrected V1.1 version with the composite output fixed. It appears to work nicely including the composite output!

Are you sure? yes | no

gkaufman wrote 08/20/2020 at 13:22

Thanks to Williad for spotting the reversed connections to the composite output. I missed that as I don't use the composite video. I have generated new gerber files and have upload as V1.1 Be aware that I haven't had a batch fabricated and tested yet - but will shortly. If you have had boards fabricated from the V1.0 files you could use a short "pigtail" and mount the composite video off of the board with the correct polarity.

I'd consider sharing the Kicad files, I just want to avoid seeing the boards re-labeled and sold commercially. I'd also rather include any improvements back to this forum if possible. Contact me privately if you do have a need for the files.

Are you sure? yes | no

williad wrote 08/07/2020 at 11:02

Hi Gary! I had some pcbs made for this project. However when I tried to get a suitable RCA socket for the composite output I found what looks like an error in the Rev 1.0 pcb. It looks like the earth is connected to the centre pin of the RCA jack & the Video is connected to the metal outer of the RCA. If you see this comment please can you check this. Thank you.

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Doug wrote 07/04/2020 at 13:06

Nice work! Would you consider sharing the Kicad project files? This would aid mods to use alternate parts if needed, or other customization.

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ASCII Video Terminal

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