Model II Warning from 1980

written by Matthew Reed

This somewhat alarming warning appeared in the June 1980 issue of 80-U.S. Journal:

The video controller of the Model II is mapped to I/O port 255 (FFH). If an output is made to this port with a value of less than 25, there is a very good chance that YOUR VIDEO WILL BE DESTROYED! We don’t know why this happens yet, but we do know that about a dozen of them across the country have been blown in this fashion, resulting in an expensive repair bill. If for any reason your screen should go blank unexpectedly, accompanied by a very high-pitched whining noise, you have less than 7 seconds to TURN THE COMPUTER OFF! Don’t worry about having disks in the drives or anything. Even the data you lose is cheap compared to having to do without a computer while it is being repaired.

A “value of less than 25” isn’t very specific, but I don’t imagine anyone wanted to test it by destroying another Model II!

I was curious, so I took a look at the Model II Technical Reference Manual. According to it, a write to port 255 selects several different functions:

  • Bit 7 disables or enables video memory
  • Bit 6 enables or disables the video display
  • Bit 5 disables or enables the real time clock interrupt
  • Bit 4 selects between 40 and 80 character mode
  • Bits 3 to 0 select one of sixteen different memory banks

Writing a value of 24 (“less than 25”) would enable the video display, disable video memory and the real time clock, select 40 character mode, and select memory bank 8. None of that sounds too dire.

I also found this in the Model II Technical Reference Manual:


When performing a test that includes turning the CRT on and off, do not leave it off for more than three seconds or damage to the circuitry may occur.

There are some similarities between the two warnings, but the bits involved don’t seem to match up. I wonder what the full story was.

Categories: General


George Phillips says:

I recall similar warnings with regards to Hercules graphics cards on the PC. I think it was something to do with setting the display width or height too small. This made the horizontal or vertical frequency too high leading to some kind of analog burn out in the CRT circuitry. Presumably turning off the video on the Model II leads to invalid CRT controller values.

Digging into the Model II schematics might be illuminating.