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5

Current loops are better for signal integrity for long runs because they are generally only "grounded" to a fixed potential at the current source. This means that there's no "ground loop" for stray magnetic fields to interact with. For electromechanical devices this is easy to implement: switches and relays don't need a common "...


5

a company like DEC that made both the computers and the terminals Actually a lot of DEC computers (PDP-8, PDP-11) had teletypes as I/O devices. Teletypes used 20ma current loops so DEC needed to provide such. Only later did DECWriters and the VT series of CRTs become the usual I/O.


19

TL;DR: It's a classic case of technological advancement vs. installed base In the early days of electricity-based communication (i.e., telegraph and later TTY) there was no way to detect a voltage and, when needed, amplify it. Only current flowing in a closed circuit could be detected reliably—by having it run through a coil which in turn moved a lever—and ...


8

The advantage of RS-232 was that it was a formal standard that defined the electrical interface between equipment, down to handshake signal usage, voltage levels and connector pinout. Even though devices only implemented a relevant subset of the standard, and the implementations did vary and were sometimes not directly compatible, in general, only a simple ...


16

The current loop goes all the way back to classic telegraphy. If there's current flowing, then that's one state. If there's no current, then that's another state. It's as simple as it can be. You don't need to manipulate voltages. That's the key. Just turn a literal switch on and off. It also has problems. Current losses are heavy even in short ...


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