420milliampereloop

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The last upload was August 2025.

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Chuck Walla said:
The bottom left 5 slots are a current loop alarm system. It's probably fire but may be intrusion. There are two live alarms and future expansion blanks for three more. I think the brand name was something like, "Ademco." It's not readable here. I can't find any similar photos yet.

Center left is a wired voice intercom. I think it's made by Ektacom. There's a similar panel in one of the Chula Vista gate shacks.

Top left may be a newer fire alarm central station panel. It looks to be 4 to 20 milliampere current loop. Why else would they have meters on it?

Right is surveillance cameras. The matching set at one of the gate shacks had what I believe to be a half-inch, reel-to-reel video recorder. Helical scan but not DVD quality.

There's a similar rack in one of the Chula Vista Plant gate shacks.

Current loop alarms, in those days, ran 125 volts DC. They had a scheme that varied by company. Made-up example: +4 milliamperes was "normal." "Alarm" was -12 milliamperes. If the circuit failed: "trouble" was 0 milliampers.

Gate shack: [https://www.flickr.com/photos/sdasmarchives/55115880443/]