Rebuilding a 1995 GPS Time Server with Raspberry Pi and Python

A 1995 TrueTime XL-AK GPS time server has been rebuilt around a Raspberry Pi 5, and the writeup was published on jeffgeerling.com. The original unit was bought in June to learn about the history of GPS-based time. Sixteen days later, a similar GPS time server took down Australia’s cell service for 12 hours.

The restomod drops a Pi 5 and a GNSS HAT with a u-blox NEO-M9N into the chassis, then drives the vintage Densitron 16×2 LCD and the bicolor status LED from the Pi’s GPIO. Chrony and gpsd handle stratum 1 NTP duty using PPS. The build is reversible: two connectors swap the original TrueTime hardware back in.

Much of the tuning work went into oscillator stability. Insulating the bottom of the Pi, where the crystal sits, enclosing the board, running a fixed fan speed and setting force_turbo all helped. PPS interrupt handling was pinned to CPU3. The server also serves RFC 867/868 Time and Daytime, plus AppleTalk time for vintage Macs through timelord. Wiring inside the case was done with 30 AWG wire wrapping, since there was no vertical clearance for Dupont connectors.

See the video below and more on jeffgeerling.com and code on GitHub.

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