CMS:Number Three Electronic Switching System

This page is about specific switch in the Connections Museum. For the switch in general, see Number Three Electronic Switching System.

The 3ESS at the Connections Museum while running

Volunteers æstrid and Colin working on the 3ESS using an oscilloscope and a logic analyzer.

The Number Three Electronic Switching System (3ESS) is an electronic switching system on the second floor of the Connections Museum. It was originally installed in 1978 as a Community Dial Office (CDO) in the village of Crosby, Washington. It contains a custom tape drive emulator created by volunteers at the museum after the original tapes began to fail.

History

In its early years at the museum, the 3ESS was cared for by volunteer Rich Barger and æstrid Smith. It was left running 24/7 at that time, as the tapes had deteriorated beyond repair, and the capstan in the tape drives had turned into goo. Despite installing new capstans in the drives, the tapes themselves were in such poor condition that reading them in the drives was impossible—any attempt to do so resulted in broken or hopelessly tangled tape.

In roughly 2012, the computers froze for unknown reasons, necessitating a reboot of the system. After the reboot, the generic (the ESS's main program) failed to load from the tape. After some troubleshooting, the machine was powered down indefinitely.

Restoration

Tape Emulator v2 as installed in the museum's 3ESS

In 2019, volunteers began an effort to restore the machine to working operation. This was a significant undertaking, which required disassembling a damaged tape, and recording the data as an audio track using a custom head block mounted to a reel-to-reel tape machine. This was completed by volunteer Dave Dintenfass at his recording studio, Full Track Productions.

After the audio was captured from the tape, volunteers æstrid Smith, Andrew Sentman, Matt Mullins, Colin Slater, and Sarah Autumn worked to turn the audio waveform into a set of binary files that could (in theory) be transferred into the 3ESS's central control. Colin created a "tape emulator" board that replaced tape machine 0 in the switch, and read the binary data from a microSD card.

Once that was mounted in the machine, a year of debugging took place, where Colin and æstrid worked to reverse-engineer the tape commands and understand what the system needed in order to boot. This proved to be a very complex task, as some of the required documentation was missing entirely. This difficulty was further compounded by not having any available debugging tools other than a logic analyzer and an oscilloscope.

In 2021, the machine booted successfully for the first time in 9 years. It is now demonstrated to visitors weekly, on Sundays.

Technical information

Power

The 3ESS has a custom powering solution. As originally manufactured, the 3ESS would cause a large inrush of current when turned on, lowering the voltage of the power supply. This was acceptable in practice because 3ESS units would rarely be powered cycled; they were designed to run for months or even years without turning off. However in the museum, the switch is turned on only on Sundays for tours or when being actively maintained by volunteers. The voltage drop from the inrush current would negatively affect the DMS-10 which shares its power supply, even causing the DMS-10 to crash. To solve this, volunteers installed a breaker system which allows smaller portions of the machine to be turned on individually, which draws less current and maintains the power supply voltage.

There are also components on the 3ESS which are sensitive to being turned on and off many times. Originally this would not have been a problem because the 3ESS was rarely power cycled, but in the museum configuration there are golf cart solenoid relays and large resistors between the power supply and the switch which handle the large current changes and protect sensitive electronics from wear and tear.