Cheap PCs from the 90s and late 1980s often had a digital display that indicated the CPU speed. They were kitschy, and I can’t say I’ve ever seen a brand-name PC with one, but many cheap cases made overseas had them. That meant the computers that came from the little clone shop down the street probably had them.
What digital speed displays did
When you turned on a PC that had one, the digital display lit up, showing the computer speed, measured in MHz. Then, when you pressed the turbo button, the display changed.
The display provided two functions. First, it showed off the PC’s full speed. Second, it provided a visual indicator when you slowed the PC down for compatibility reasons. Some older software would malfunction on faster PCs.
Some people liked them, especially if their PC was fast enough to be worth bragging about. Lots of people thought they were tacky. I remember a Mac snob who lived down the hall from me in college asking me why all PCs had a cheap-looking digital speed readout. Not all PCs did (mine didn’t), but when you want to look down on others, anything’s true all the time once you see it twice.
Why digital speed displays faded away
These speed displays became less common in the late 1990s as 32-bit software gained prominence. Speed sensitivity was a problem mostly for 16-bit DOS applications, and as time wore on, more and more people migrated to newer, 32-bit Windows applications, The speed displays disappeared before turbo buttons did, but by the year 2000, it was getting difficult to find a case that had a turbo button at all.
Today the cases have a following again. The five senses play a strong role in nostalgia, so people remember those green displays. A lot of people had one or knew someone who did. In 1996, a case that had one usually was cheaper than one that didn’t. Today, an old case with a digital display is probably worth more than one that doesn’t.
How digital speed displays worked
There wasn’t any intelligence in the digital speed displays that sensed the actual speed of the computer. They just lit up one set of LEDs in one mode, and a different set in another one. The display depended on a two-wire power cable from the power supply, which usually was wired to one of the molex cables for disk drives. The actual display was configurable via a set of jumpers.
Early cases had jumpers to select one of a set of common speeds (usually 8, 12, 16, 25, or 33 MHz). These were easy to configure, but not very useful once PCs reached higher speeds. Later cases had fully programmable displays. They had a matrix of jumpers that controlled each segment. Leaving a jumper off entirely turned the segment off all the time. Putting the jumper in selection 1, 2, or 3 switched it from being on in normal mode, turbo mode, or both.
This meant you had to work out which segments needed to be on in which mode, then set the jumpers.
Figuring out how to configure the front display was one of the hardest parts of building a PC during that era. I didn’t know very many people who had the patience to do it, and I imagine most clone shops kept cheatsheets.
Information about these digital displays is hard to come by these days, since few people saved those instruction sheets, if they ever got them in the first place. Here’s a broad guide that applies to the majority of those displays, and an archive of documentation for 18 different displays.