Ken St. Cyr Gives a TRS-80 Model 100 a Serious Speed Boost with Steve Adolph's "Turbo Mode" Mod

With a clever clock-doubler, you too could have the fastest Tandy TRS-80 Model 100 around — if you don't mind a few compatibility issues.

Gareth Halfacree
2 months agoRetro Tech

Vintage computing enthusiast Ken St. Cyr has turned a staple of 1980s portable computing into a powerhouse — relatively speaking, anyway — with a modification that boosts the TRS-80 Model 100's clock speed to a breathtaking 5MHz.

The Tandy [Radio Shack] Model 100 [is] a beautiful piece of laptop history running at a relaxing 2.5MHz," St. Cyr says by way of introduction to the modification, "but I think we can do better. Today, we're going to make this device do something that it really wasn't designed to do."

A clever "clock doubler," plus a few extra components, can double the performance of a 1983 Tandy TRS-80 Model 100. (📹: Ken St. Cyr)

Released in the US back in 1983, the TRS-80 Model 100 — given its moniker for the company which made the original desktop version, Tandy, and the Radio Shack stores through which it was sold — took Tandy's popular TRS-80 microcomputer design and made it portable. First launched in Japan by Kyocera as the Kyotronic 85, the all-in-one device packed an eight-bit microcomputer with 8kB to 32kB of RAM, an eight-line 40-column display, and a 56-key keyboard into a battery-powered portable roughly the size of a textbook.

As St. Cyr says, the original design had the CPU — an Intel 8085, first released in 1976 — running at 2.4MHz. The Intel 8085, though, is capable of more: the first model could be clocked up to 3MHz, and later releases could hit 6MHz. As a result enthusiasts have designed "clock-doubler" boards that install within the TRS-80 Model 100 and allow the CPU to run at 5MHz, or a little over twice its stock speed.

"There are a couple of caveats," St. Cyr notes of the upgrade, which uses a design released by Steve Adolph on the Bitchin' 100 fan site. "Steve created a custom circuit board that piggybacks on top of the CPU inside the Model 100. If the CPU goes too fast, the other components may not be able to respond quickly enough before the CPU moves on to the next operation. So, in order for this mod to work we need to replace the ROM chip with a faster one."

"Overclocking the CPU will [also] have a downstream effect of overclocking the UART," St. Cyr continues, "and that'll break the serial data exchange timings in the RS232-C port and the modem. So, to work around this we need to inject the system's original clock speed into the 81C55 chip at Pin 3."

Once installed, the mod is invisible to the host system until "turbo mode" is engaged by typing OUT 85,1. When activated, "turbo mode" provides a decent speed boost to a range of CPU-based tasks — though it's not without its problems. "Starblaze 100 was a game made by Radio Shack back in 1983, [but] it doesn't work well. This is a known issue: no-one really knows why Starblaze 100 doesn't work with the 5MHz mod."

For those who use their TRS-80 Model 100 for things other than blasting aliens, though, the mod is detailed in full on the Bitchin' 100 website — complete with links to download the patched ROM images and the PCB design files.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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