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In the early 1980s my primary school was the proud owner of a Commodore PET 2001.

There was a commonly held belief among the young geek fraternity that there existed a BASIC command—perhaps a POKE—which would cause physical damage to the machine. I don't know where this story came from but I do remember hearing it from sources outside the school and that it specifically involved the PET.

Does such a command exist, or was this just an urban myth?

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    Wasn't this the basis for the "Halt and Catch Fire" term? – cbmeeks Dec 7 '16 at 21:11
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    I'm pretty sure that term pre-dates the PET. – Flup Dec 7 '16 at 21:46
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    I don't know if this machine specifically was affected, but I've heard that some early Commodore could be damaged by POKEing a serial port control register to select an external baud rate clock (rather than its internal baud rate generator). This external input was left floating (bad design!), which on a CMOS chip tends to result in oscillation at the maximum frequency possible. This supposedly caused the chip to burn up. – jasonharper Dec 7 '16 at 22:35
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    @cbmeeks, the Jargon File traces it back to the System/360, and notes a similar EOU control character for the ASR-33 teletype. – Mark Dec 8 '16 at 5:38
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    Adding as comment not an answer as I don't have verification for this, but I had been told it was possible to program the video chip to collapse the entire display to a single scanline. With the CRT scanning the same line over and over it could permanently burn the phosphor relatively quickly. – Richard Downer Dec 8 '16 at 11:51
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POKE 59458,62 was a trick, sometimes called "Fast Print," used to increase screen refresh rates on older PETs. This page describes the trick succinctly:

the system no longer waits for the video sync signal, thinking that it's always present, and updates the screen as fast as it can.

In later models this POKE could cause problems. The new video circuitry behaved differently, and the trick could cause the CRT's flyback circuitry to generate excessive voltages and possibly damage the monitor. A more detailed analysis of this is here. The PET 2001 seems to be one of the models that would not be damaged by the Fast Print/Killer Poke, but later PETs might be (this FAQ seems to confirm this).

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    Great first post, and welcome to Retrocomputing! – JAL Dec 7 '16 at 22:51
  • I have a PET 8032. I wonder if I should try this? ;-) – cbmeeks Jun 12 '17 at 14:30
  • @cbmeeks If you want to get rid of the monitor, I could take it off your hands. But please don't destroy it, unless it's one of those fancy modern ones. – wizzwizz4 Mar 11 '18 at 17:18
  • @wizzwizz4 sorry...the PET (and monitor) will be taken away from me when they pry it from my cold, dead hands. lol. I bought it back when eBay had great deals (~$200 SHIPPED). – cbmeeks Mar 12 '18 at 18:50
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This jogged me memory about a pretty well known problem with early PC video control hardware that could, if programmed with really incorrect video timing, result in damage to the flyback power supply in monochrome monitors.

Here's a link to a more detailed exchange on the subject of monochrome monitor damage from software: How did this program burn out two monitors?

This post states:

This is an old known bug. You can burn out the IBM monochrome monitor by stopping the horizontal sweep while keeping everything else running, and the Hercules card gives you enough control to do this under software control. The video chip lets you select the horizontal and vertical sweep rates independently, and zero rates are possible. However, the horizontal sweep is used as the oscillator for a switching power supply, as is typical in TV circuits, and with the sweep rate at 0, DC flows through a coil with high inductance but low resistance, producing an excessive current that burns out the coil.

Part of the problem is that the IBM monochrome monitor is a design lifted from an earlier, pre-PC product line, the IBM Displaywriter, and in that product, there was no potential vulnerability of this type.

and this post states

If you tinker with the registers in the 6845 CRT controller chip on the video board, you can program in a horizontal scan rate that will overheat the power supply in the monitor. The reason being that a monochrome monitor is designed to only work within a very narrow range of horizontal frequencies. The problem seems to come from accidentally programming a 6845 on a herc display with values that are appropriate for a CGA monitor. Such a mistake is pretty easy to make whilst tinkering with graphics programming and accidentally using the wrong setup from a library. Doing herc graphics with Borland's BGI system seems to be pretty safe, as I still have my herc monitor intact.

Several people reported that a screen-saver program called "BURNOUT" fried hercules boards, rather than the monitor. This surpirses me, but several people were quite emphatic that it was the board that went south.

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You can play with VSync on many systems and cause that situation, but it's unlikely to create damages; on most systems, from the Atari 2600 to PCs until the 486 era, you had ways to do this. There was a French computer called the TO7 that had a module designed to make overlays on top of a composite video signal. This is the only computer I know of, from this era, where it was possible to create a physical damage (to the module), from improper programming. I haven't witnessed it, but I remember that it was explained in depth somewhere. Edit as I forgot: the NES could also be damaged with improper programming of the PPU.

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    Welcome to Retrocomputing! Don't forget to check out the tour to earn your first badge. I hope you stick around to answer more questions, your expertise would be quite valuable here. – JAL Dec 7 '16 at 22:49
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Yes, also the later 4000 models. The POKE may have been different with those. It would mess up the Vsync and the screen would be all warped and distorted. Completely useless for playing AFO.

[In fact I even tried the poke on my 8296 with connected screen. As a late model I assumed it has the fix, but kept my finger close to the reset button, though. As far as I can see from the screen the CRT now simply does not get any sync anymore, the screen just moves.][1]

A fix incorporated into the 4000 and later series to correct a problem with the 2000 series that drastically slowed down the PETs when graphics were displayed.

Early on type in patch was discovered for the early PETs. It seems that the video chip was able to be accelerated by poking a value to a specific address. The problem came when the new fixed PETs arrived, it seems that by poking the value to the same address in the newer PETs would cause the video chip to accelerate to the point they would overheat and destroy themselves. Thus the term 'killer poke' soon became a well known legend in PET history.[2]

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The computer lab at Bradford District High School consisted of about eight PET 4032's connected to shared printers and disk drives.

Every so often the "killer poke" would come up, the last time I recall I think was in the letters section of Compute! Invariably, shortly thereafter someone would try it. The result was that the screen would go "wonky" as if you had incorrectly adjusted the horizontal hold control on an analogue TV, sort of "tearing" as opposed to the ordered rolling of a maladjusted vertical hold. No one was that crazy though, and always turned off the machine within a few seconds.

The year I left, one of the 4032s died, and everyone swore up and down it was due to the killer poke being left running overnight by mistake. I cannot personally confirm this. I returned later that summer and the machine had been fixed, whatever the original problem happened to be. That is when I saw Space Spores for the second time (the first was at Ontario Place).

This was also the summer where the business classes ICON computers all died when a trojan RMed the entire hard drive. Apparently, it was hidden in a BASIC spreadsheet that the teachers always ran in their prived accounts.

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