[cctalk] Re: Hot Video Shift Register on VT100

Hi Rob Distrust of the instrumentation is always prudent, but an open mind is also always merited. As your 74S299 is a standard footprint, you have the option of logic types other than F as alternates. The two things it may be worth minimising are slew rate, which will cause ringing, and switching current, which will cause ground bounce (due to inductance in the ground path). Subject to necessary drive capability my first thoughts for house trained 5v logic would be LS or HCT. There is a smorgasbord of logic types - TI have some usable summaries in pdf, including timelines. Good Luck Martin -----Original Message----- From: Rob Jarratt <robert.jarratt(a)ntlworld.com> Sent: 24 December 2025 18:15 To: Martin Bishop <mjd.bishop(a)emeritus-solutions.com>om>; 'General Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org> Subject: RE: [cctalk] Re: Hot Video Shift Register on VT100 I found some time to look at this again today. A few answers below. To I pre-emptively bought an oscillator (24MHz and not 24.07342Mz) and tried it on a breadboard. This gave me the same signal with a negative spike on the falling edge. Although I don't have a ground spring I managed to shorten the ground connection considerably and this did indeed clear up the signal and it looked a lot squarer, without the negative spike. I lifted the original oscillator, but the leads on it are too short to work on my breadboard so I can't verify the same behaviour, but it seems highly likely that you are right that the negative spike is not real. That leaves me with the 74S299 still getting hot for reasons I don't understand. Maybe it is just the design, but the same chip in a VT102 doesn't get nearly so hot. Someone else suggested replacing it with a 74F299, so maybe I will try this now that I have socketed it. Regards Rob following the