How to: DIY 40/80 column auto softswitch
Following my other DIY (Do It Yourself) hardware project (keyboard beeper circuit) I thought I'd post info about another board (and very useful IMHO) I made a while (some decades!) back: a 40/80 column autoswitch for use with a Videx 80-column card (it probably works with other 80-column cards as well). I don't know its source or have any more info than what I'm posting below, but can say it works great.
I've used it with an Apple II+ clone along with a Videx Videoterm clone. It plugs into the Apple II F14 position IC socket and in turn takes the IC from there in its own "through" socket .
What is it?
The Videx 80-column card has its own video output, and so does the Apple II (40 columns). After selecting the desired output in software (i.e. PR#3) you also need to change the cabling to the monitor. This is traditionally done with a mechanical switch, but the autoswitch eliminates this completely so any chosen video output just appears on the screen by itself. Two LEDs indicate which column-mode the switch is in.
- 3904 transistor (2 pcs.)
- LED (2 pcs.)
- 16-pin wire wrap IC socket (2 pcs.)
- D4011 IC
- 14-pin IC socket (optional -for the 4011)
- 220 Ohm resistor trimmer
- 220 Ohm resistor (2 pcs.)
- 3.3K resistor (2 pcs.)
- 6-way socket
- 6-way header
- 2-way headers (2 pcs. -for inserting into Videx card and video/GND pins on Apple II board)
- phono socket (for insertion of monitor cable)
- connecting wire (suitable lengths for connecting between board and Videx card, monitor etc.)
F14 (Apple II) IC-socket replacement
As with my Apple II Keyboard beeper board I would suggest you start off by replacing the Apple II position F14 IC-socket with a wire-wrap socket. F14 should be a 74LS259 or a 9334) and is a 16-pin socket. Failing to use a wire-wrap socket (or similar -you can use two 8-pin Arduino stackable headers instead) means that the board will fit very loosely and possibly even fall over, short circuiting your computer!
Follow the instructions/photos for the Keyboard beeper (scroll down to "B10 socket replacement (Apple II main board)" on how to do this (only remember to use a 16-pin socket (or two 8-pin Arduino headers) here instead of 14-pins!
How to build/set up
The project is pretty straight forward. Back in the day I hand drew/used rub-on circuit decals and painstakingly etched my own PCB, but if I were to do it today I'd redraw it in some CAD software such as Eagle or KiCAD, then have it professionaly made (if someone were to do that here I suggest sharing the design files by uploading them in this thread).
Be sure to place the two ICs the right way. As for the LEDs -you're free to use different colours for each mode (40 and 80-columns) or the same. It makes no practical difference but might be handy for troubleshooting/indication purposes or visually appealing.
The 6-pin connector has video signals from (a) the Videx 80-colum card, (b) video signals from the Apple II main board and (c) the video output which goes to a (preferrably green/monochrome) display monitor. Make sure you get the polarities right!
The usual video out jack on the back of the Apple II isn't used at all with the auto-switch, but in my case I have a separate colour monitor attached to that output and use it for games, and a green monitor which is connected to the auto-switch output and is used for "serious" software (80-column word processing, CP/M, programming and so on). The best of both worlds without the hassle.
The two trimpot resistors on the soft-switch allow you to adjust the video levels so you can match both modes instead of having to reach for the monitor's "brightness" control each time you change a mode. The video output trimmer on the Apple II (close to the game port socket) makes no difference here -it only adjusts the video level for monitors attached to the rear Apple II video output jack.