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According to https://en.wikipedia.org/wiki/VT52

Like other early DEC terminals, the VT50 series were equipped with both an RS-232 port as well as a 20mA current loop, an older serial standard used with teletype machines that was more suitable for transmission over long runs of twisted-pair wiring.

"more suitable for transmission over long runs of twisted-pair wiring" sounds like a really big advantage! Yet it seems the 20mA current loop was replaced by RS-232. Why? What advantage did RS-232 have to outweigh this disadvantage relative to the older technology?

(Eventually there would come a time when 'RS-232 is the industry standard' would be decisive. But for that time to come, it must first have passed through some years when it had to appeal on technical merit; a company like DEC that made both the computers and the terminals, would have no reason to care about the standard, if it could get better signal transmission by making both computers and terminals use a different format.)

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    The KIM-1 originally only supported the 20ma interface. – Will Hartung Feb 4 at 17:50
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    Modem signaling. – another-dave Feb 4 at 18:06
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    Also, for DEC in particular, it wasn't always a 'terminal' connected to an async line - it could be some random piece of customer-provided equipment. DEC had an interest in making sure its computers supported the interfaces its customers needed. Typically, comms interfaces could be fitted wirh 20mA or EIA line cards. – another-dave Feb 4 at 18:09
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    Guys these are answers, not comments – OmarL Feb 4 at 18:55
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    4-20 mA current loops are still used in industrial systems for their ability to power remote sensors without needing a separate power line, and their better noise immunity compared to voltage-mode signalling. I don't know much about their historical use in computers, but they're not at all a rare sight in anything to do with PLCs and other industrial control systems. – Hearth Feb 5 at 4:08
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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 amplified as well—by having that lever move a switch that closes another (next) circuit.

At the time voltage based interfaced became possible it stand against a huge installed base - and existing equipment - using current interfaces. Computers, the ones promoting voltage based most, had to arrange with existing hardware, until gaining enough of a market to make it worthwhile to invest in new production.


The Long Story

[All of the following is quite simplified as the steps taken and the associated learning curve was quite complex]

When telegraph became a serious application, there was only one device that could transform an electric signal into something useful, a coil: When current is applied it generates a magnetic field, able to pull some magnetic lever, which in turn can be used to operate something, like a bell, a pen or whatever (*1).

The magnetic field generated is independent of voltage, but directly proportional to the current flow (*2). To make it work, a certain amount of current is needed, such as 50 mA. As soon as that current flows, the coil will pull its lever. Since pulling with more force isn't a big issue, this also provides an inherent handling of overcurrent (within reason).

So much for the receiving side. With the sender, voltage comes into play, as the amount of current delivered over a line is directly proportional to its length (its resistive load). So it was possible to calculate the needed voltage to for a given line length, which then was fixed for that relation. To simplify, companies tried to use only sections of equal length. Sections could be chained by combinations of coils and switches operated by the lever, switches that closed the circuit of the next section. Today we might call this a digital amplifier, but back then they choose relay, like the familiar stations on a stage coach line.

So genuine telegraph and teletype communication was current-based. Soon many lines were built in a real boom, setting up a current-based infrastructure. Of course there were many other improvements, from power generation to power control, insulation to wire size and quality. And not least an incredible variety of devices based on coils pulling levers—the integrated circuits of the 1890s.

Current-based networks rule digital communication, and a huge number of users attract a good number of manufacturers supplying teletypes, teleprinter and other related equipment. Current-based interfaces were the industry standard for such devices.

This is essentially the situation at the advent of computers. By now voltage-based interfaces were possible, not least due to the transistor which drove computer development as well. They are simply the way a computer work internally, so using them with external components would have been a great thing—except, all easy-to-get terminal devices, aka teletypes, worked on current loop.

So the logical solution was simply to add interfaces that change voltage-based (serial) signals to current loop and back. Of course that additional effort is not really wellcome, so establishing RS-232 was quite appreciated and by the mid 70s essentially done.


In Detail

"more suitable for transmission over long runs of twisted-pair wiring" sounds like a really big advantage!

Not really. Or only with sufficient effort. After all the primary output of (most) computers was voltage-based.

Yet it seems the 20mA current loop was replaced by RS-232. Why? What advantage did RS-232 have to outweigh this disadvantage relative to the older technology?

It simplifies interfaces on the computer side—especially if both sides are using the same, voltage-based logic. No converting interface needed. Everything gets simpler with voltage-based interfaces—not to mention that they also allow higher transmission rates.

a company like DEC that made both the computers and the terminals, would have no reason to care about the standard, if it could get better signal transmission by making both computers and terminals use a different format.

The point is rather that even 'a company like DEC' started out small, so not really able to build a whole landscape right from start. In addition, there's the pesky customer, one that wants to use existing terminals, or ones available at a lower price from third party, or simply what they already have.


*1 - The folks back then became very innovative what to do with the little force a tiny lever pulled by a coil can operate.

*2 - Yes, the amount of current is of course related to the voltage applied, but that's beside the point.

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    re not really a love child - I think you might have the wrong interpretation of that term. To my mind, hooking up voltage-mode devices to current-mode circuits is very much a love child, which is the polite way in English to say "a bastard". – another-dave Feb 5 at 4:02
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    Another interesting point that might be worth mentioning is that in the early days of telegraphy, the most practical batteries would degrade quickly whether or not current was drawn from them, so it was common to have all telegraph keys wired in series, with a bypass switch on each one, so current would flow through the line when idle. When a transmission was going to arrive, current would drop, then the data would be sent, and then the line would go back to carrying current continuously. – supercat Feb 5 at 16:05
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    Re, "... so current would flow when idle." Anyone who wants to follow that thread far enough via Google search eventually will learn why the line states traditionally are called "mark" and "space." – Solomon Slow Feb 5 at 18:15
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    Also remember that DEC didn't start out making their own terminals. Most early PDPs used Teletype brand terminals. Even after DEC started making VTs, teleprinters were the installed base, and they were very much RS-232 devices. – Warren Young Feb 5 at 18:39
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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 conductors. You have to transfer enough power to run the equipment on the receiving end, such as the electromagnet in the receiving relay. Also, if you're driving more than one device, you have to account for that. For example, the first transatlantic telegraph cables had to run at tens of thousands of volts to get enough current on the far end. As you go faster, it's also hard to get sharp edges of a signal without ringing, or it being "smoothed" out by transmission line effects.

By comparison, RS-232 is all about voltage. The tiniest amount of power can signal either state, assuming it's at the correct voltage level. A few milliamps can drive the input stages of many amplifiers all in parallel, so one RS-232 transmitter can be used in multidrop situations sometimes.

You can also use longer and thinner conductors without major consideration. Parasitic capacitances and resistances are much less of a concern. You can clock it much faster. For short wires, you can get into the kilobaud before you have to really worry about things like ringing. Voltage-based signals can be balanced, and run on twisted pair, offering considerable noise immunity.

The advantages of this in principle were understood early on. It's much the same trade-off as between a baseband analogue signal and an modulated analogue signal on a carrier. More robust, but more complex. To do it, you have to be able to easily manipulate voltages, and that means electronics. Vacuum tubes or transistors. It was really the arrival of transistors that made RS-232 practical.

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    At least in the 1950's - 1960's era, current loops were also used for plant automation, including the large chemical plant my father worked at in that period. While RS-232 was introduced in the early 1960's, there was significant installed base of current loops. – Jon Custer Feb 4 at 22:42
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    @JonCuster 4-20 mA current loops are still standard in a lot of plant automation today. They often have digital data superimposed on them now, but the primary signal is still analog. For more complex data transfer where ethernet hasn't taken over, RS-485 is used. – Dranon Feb 5 at 2:30
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    20mA loops have much longer range than RS232. – Jasen Feb 5 at 5:37
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    When I was fixing communications kit in the 80s, taxi offices (as an example) were often quite a distance from the actual radio (over 1/2 mile in one case) and they used 4-20mA current loops for transmitter control. Those loops often had hundreds of volts at the drive end. – Peter Smith Feb 5 at 11:10
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    MIDI for connecting electronic musical instruments is also a current loop, where the sending device drives the LED in an optoisolator at the receiving device, so the sender and receiver can remain electrically isolated. – nekomatic Feb 5 at 13:47
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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 breakout box to wire handshake and data signals correctly with jumper wires was needed to connect almost any two pieces of equipment together.

Comparing that to the de-facto 20mA current loop interface, it only contained a single serial data signal without handshake signals, so only thing that could be detected on both ends is a break in the current loop wire. Connecting two pieces of equipment together was not as simple as connecting a pair of wires or connectors between them. The connectors varied, the pinouts varied, and the electrical I/O varied. Sometimes the current-limiting was in the transmitter, and sometimes in the receiver. Matching equipment together requires far more detailed knowledge of their internal electrical implementation to connect them properly, as there can be damage if neither end implements current limiting, or it may fail to work if both ends implement current limiting. External components, such as resistors, could be needed to match the two devices together.

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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.

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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 "ground" connection.

It's more difficult with electronics, as it is much more convenient to have a common "ground": isolated interfaces are more complicated. With electronics at only one end of the link, as when a Teletype was attached to a computer, it was OK. The common "ground" could be the computer's common, and the electromechanical Teletype needed none for its interface.

But with electronics at both ends, a different approach was desirable, and RS232 was an early attempt, with large voltage swings and hysteresis to aid signal integrity. But the longer the link, the less well this works.

In an extreme case, I've seen an RS232 interface board with a hole blown in it by the current around the ground loop due to a nearby lightning strike! The link was about 50 meters long, but current loop links can be much longer.

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