I find the RS-232 standard fascinating as it's one of the standards in computing that's survived--and is still used--for well over 50 years. The only major development has been a change in voltage from 12v to 3.3v in most applications, but with 12v still available and being used.

The Wikipedia article on RS-232 tells us the standard was developed in 1960. The Bell 103 modem, the first one (acoustically coupled), was released in 1962.

That article also says the very first devices were "electromechanical typewriters" - which makes sense. The famous ASR-33 was released in 1963, but there were models beforehand. The Bell 103 couldn't have then been the first RS-232 device. So what was the first?

  • I don't follow why the Bell 103 "couldn't have been the first" - can you expand on that? Prior to EIA-232, teletypes used 20mA current loop, which only carries data signals. Presumably the switch to EIA-232 coincided with availability of commercial modems for use on the PSTN. – another-dave Sep 5 '20 at 0:54
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    Note, there was a Bell 101 modem, commercialized in 1959 out of work on SAGE. It was 110 baud, which matches KSR/ASR 33 speeds. I can find nothing about it. It predates EIA-232 as an approved standard, but I suppose it could have implemented a protocol which was subsequently standardized. – another-dave Sep 5 '20 at 1:08
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    Given that RS-232 is basically just the traditional teletype current loop translated to voltage levels plus some control signals, it may be very hard to narrow down the first device that was /practically/ compatible with RS-232. It's relatively trivial (even with 1960s electronics) to convert between the two. The first RS-232 devices may have been a retrofit or custom option of some kind. – RETRAC Sep 5 '20 at 6:05
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    That question cannot be answered in a definitive way. It shows a clear misunderstanding on how standards used to evolve: RS-232 was designed and standardized after a significant number of devices implemented serial comms and it tried to align these implementations towards a common set of parameters. These devices obviously can't have been designed after the standard, because it didn't exist yet . (Typical chicken-and-egg-question). ..... – tofro Oct 8 '20 at 8:56
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    BTW: There's never been a "change in voltage" of RS-232. Devices that use LVTTL for serial transmissions are simply not RS-232. – tofro Oct 8 '20 at 9:40

What was the very first device to have an RS-232 serial port

The question is a bit misleading, especially when tying it to 1960.

RS-232 is a standard. Standards are usually made after something has been invented and used first. Their purpose is usually to either unify a varying landscape or turning de facto usage into standard (*1). Think of bolt dimension, tube sizes or screw driver heads.

RS-232 is such a standard codifying and unifying existing industry usage and terminology.

As such there is no real 'first' that can be pointed out without doubt. It's kind like a quantum problem. It's like the cat in the box. prior to opening the box, noone knows if the cat is alive or dead. Or in this case, there can't be no device adhering to RS-232 before it got recommended, but as soon as it was published, a lot of existing devices were compliant and instantly became RS-232 devices, or not ... Getting a common classification is the whole purpose of such a standard


Serial communication using asynchronous is way older. The beginning of what after several steps became RS-232 was Baudot's "Système de télégraphie rapide" which in many ways did forshade upcoming information age - for example by being the first to use a binary representation for encoding and transmission (*2). Like telegraph before it used a current loop for the signals.

Development evolved over the next 100+ years. While Baudot's first machine was semi synchronous (the operator had to type the next character every time the keyboard got release from the previous character, later became truly asynchronous.

While original current was around 60 mA, it got lowered to 20 mA when the 7 bit teletypes got introduced. All still asynchronous over a single line.

With emerging switching and amplifying voltage driven communication became a thing. Various companies developed a great range of interfaces/devices, using different voltages and additional signals.

EIA generated RS232 to unify these into using the same name, workings and meanings.

That article also says the very first devices were "electromechanical typewriters" - which makes sense. The famous ASR-33 was released in 1963, but there were models beforehand. The Bell 103 couldn't have then been the first RS-232 device. So what was the first?

Hard to say, as RS-232 covers a wide range of devices already in existence in 1960. There is no minimum requirement for being either duplex or using any of the control signals. A single pair carrying a word synchronous transmission guarded by a start and a stop bit will be RS232 compliant. So essentially a (Baudot) TTY interface changing current loop to voltage level would fit - and they have been around from the 1870s - for sure including the early days of computing.

*1 - Not to be confused with (interface) specifications made up by companies/inventors/committees to define something new to be implemented later - these may as well become a standard at some point.

*2 - All the way to time multiplexed transmission.

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    Why is this being voted down – OmarL Sep 5 '20 at 7:55
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    Because it’s a long, meandering post that doesn’t actually answer the question? It might as well point to the Big Bang as the origin of the RS-232 standard. – user3840170 Sep 5 '20 at 8:17
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    @user3840170 Good point, so lets use 'Big Bang as generic answer and close everything. After all, RS232 wasn't anything specific new, but just the codification of what happened before for the purpose of industry wide standardisation to make devices interchangeable. Asking for the first device and standard compilance in this context is shrouded in a kind of quantum state. One can only measure either, device usage or standard definition. For historical purpose guess you really want to dig a bit into Teletypes first and Baudot's inventions. – Raffzahn Sep 5 '20 at 8:24
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    I think, though, that we can treat the question as asking "what was the first device to use the communication protocol that subsequently became standardized as RS-232?". – another-dave Sep 5 '20 at 15:37
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    @sawdust - The physical characteristics (wires, voltages, …) of communication are the layer-1 protocol. – another-dave Sep 7 '20 at 22:15

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