Seriously, you do not want to make that cable your earth

Network? What's that when it's at home?


On Call This week we bring you a shocking incident for a Register reader who was party to an electrical engineer's earthly delights.

"Andrew" takes us back to the 1980s, the days of DECNet, DEC Rainbow PCs, and the inevitable VAX or two.

Back then, DECnet was a big noise in networking. Originally conceived in the 1970s to connect PDP-11 minis, it had evolved over the years and was having its time in the sun before alternative networking technologies took over.

At the time, it was all the rage. "We had a load of VAX boxes and Rainbow PCs all over the office, spread over four floors," Andrew recalled. "So we installed Digital's latest and greatest routers, connected by thickwire Ethernet."

"For those who are too young to remember, this was a thick, yellow, armored coax that was electrically balanced," he explained. "That means it had a very important minimum bend radius, and you could only connect to it at marked points where the reflections peaked. You used a bee-sting to connect a coax cable."

While it was a little more complicated than the topologies of today, it did the trick. "All was fine for a couple of months. PCs were connecting to VAXes; VAXes were exchanging files and email on what was then the state-of-the-art All-In-One office system."

It was, as Andrew put it, "great stuff."

Until it wasn't. One floor dropped off the network completely. The floor below was, according to Andrew, "running like a three-legged dog with arthritis." Hardly a shining example of DECnet, and he (and a tame network engineer) were called upon to diagnose the issue.

They went to fourth floor first and fired up their test kit. Sure enough, the cable showed issues and zero connectivity to other floors. "Back to the third floor," he told us, "and the reflectometer said the problem was between floors."

So to the riser to look at the cable itself... and the panels were already off with a pair of electricians busying themselves with the wiring within.

This was a bit... odd. Andrew and his pal asked what was going on.

"Oh," came the cheery response, "we were just commenting on this new armored earth cable that you boys have installed."

"Never seen it before, but it sure is useful to be able to earth this new spur we are just about to turn on."

We imagine there was a short pause as Andrew gathered his wits and pondered the least sweary way of delivering his explanation. No, this was NOT an earth. It was, in fact, the network...

"...and if they turned on the power not only would the circuit not be earthed, but hundreds of thousands of pounds of computer equipment would go up in smoke."

"Network?" asked the electrician. "What's that when it's at home?"

Ah, the 1980s. Simpler times. These days all we need to worry about is Bob in Sales insisting he be moved from the Wi-Fi access point for fear of having his brain cooked while still spending much of his day with a smartphone pressed to his ear.

As for Andrew's nearly-earth DECnet: "The cable was ruined and had to be replaced. The next one had tags on it saying 'NETWORK – DO NOT TOUCH'."

Ever saved a user from a shocking experience with the wrong cable? Or repurposed some wiring because it was simply in the right place at the right time? Confess all with an email to On Call. ®

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