Timeline for Why was RS-232 12V?
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23 events
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| May 9, 2021 at 9:12 | history | edited | Toby Speight | CC BY-SA 4.0 |
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| May 8, 2021 at 11:17 | comment | added | Raffzahn | @SolomonSlow +/-25V was always the maximum voltage to be expected. | |
| May 8, 2021 at 10:56 | comment | added | Justme | @SolomonSlow Do you have something to back up how those voltage levels differed between different RS-232 standards? What I have gathered, all versions of the standard from from -C to -F have exactly same voltage levels defined for receivers and transmitters. | |
| May 8, 2021 at 2:13 | comment | added | Raffzahn | @SolomonSlow Tell that to all sites with extensive the inhouse cabling for terminals and printers - which was quite well part of the spec. RS-232 was intended to replace (next to) all TTY based connections. | |
| May 8, 2021 at 0:37 | comment | added | Solomon Slow | Re, "...burden of handling loss..." But, RS-232 never was meant for long-haul communication. I'm not saying it never was used that way, but what it was meant for was interfacing Data Terminal Equipment (i.e., a computer or a computer terminal) to Data Circuit-terminating Equipment (i.e., a modem). The modem and the DTE were supposed to be in the same building, if not in the same room as each other. The long-haul part of the link was supposed to be from one modem to another. | |
| May 8, 2021 at 0:29 | comment | added | Solomon Slow | Re, "RS-232...defines +/- 3..15V" The current standard is RS-232-F. Before that, there was RS-232, RS-232-A, RS-232-B, C, D, and E. I think I read somewhere that the original standard required +/- 25V signals. Later revisions narrowed the minimum voltage swing down to +/- 12V, then +/- 5V, and most recently, +/- 3V. | |
| May 7, 2021 at 16:30 | comment | added | Mark Morgan Lloyd | I disagree that RS232 had any direct connection with (electro)mechanical devices. It was specifically for interfacing a DST (modem) to a DTR (terminal), if a TTY was involved there would have been an additional interface box since these were generally current-loop devices. | |
| May 7, 2021 at 16:27 | comment | added | Maury Markowitz | If the idea was to use some common and useful level, 48V would have made orders of magnitude more sense, especially in a communications setting. | |
| May 7, 2021 at 6:51 | comment | added | Justme | The RS232 standard specifically says that the load must not be inductive, so coils should not be directly driven with the interface signals. Besides rated reveiver impedances are from 3k to 7k ohms so drivers are not expected to encounter loads outside that range. Driver only has to generate rated range from +/- 5V to +/- 15V when the rated load is connected, so unloaded receiver is allowed to go up to 25V. The receiver side must properly receive +/- 3V to. +/- 15V range which allows losses in wiring. The receiver must handle +/- 25V without damage, it does not need to work at those levels. | |
| May 7, 2021 at 6:11 | comment | added | the busybee | What are the recommended baudrates that time? I can hardly think of any electromechanical device that reliably handles pulses shorter than -let's say- 10ms, limiting baudrates to at most 110. Are there asynchronous receivers built from relays? | |
| May 7, 2021 at 1:16 | history | edited | Raffzahn | CC BY-SA 4.0 |
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| May 7, 2021 at 0:46 | comment | added | jwdonahue | True, but ~90% of the harmful energy occurs well below the peaks. It's all about thermal breakdown of the insulation on the coil wire. High current, solid state diodes probably weren't very common in 1960. Perhaps the spec was driven by the high effective response times of commodity snubber circuits? | |
| May 7, 2021 at 0:18 | comment | added | rackandboneman | @jwdonahueI the back EMF of most solenoid-like devices will far exceed 25 volts, so even back then I would assume a snubber circuit and/or freewheeling diode would be used. | |
| May 6, 2021 at 23:39 | comment | added | Raffzahn | @DavidTonhofer Cranking up voltage is the whole point of a current based transmission. isn't it? It's all about delivering the current needed to operate a solenoid at the other end. Not to mention, that, at the early days, it was easy (or better less hard) to guarantee a current than a voltage. Current only needs coils, while voltage (usually) needs amplifiers - expensive high tech before the transistor. | |
| May 6, 2021 at 23:19 | comment | added | David Tonhofer | "A current supplied in a current based interface will always reach the receiver at the same level." Except that you may need to crank up the voltage heavily to reach acceptable current levels? For a voltage-based circuit, once capacitance has been filled, you will reach your input voltage at the far end. At 9600 baud, I would say that's less energy intensive? (Also, I remember Telex lines at 40V) | |
| May 6, 2021 at 22:15 | comment | added | Raffzahn | @jwdonahue exactly that's the point about based on electro-mechanical receivers. | |
| May 6, 2021 at 22:12 | comment | added | jwdonahue | Inexpensive and robust 12V solenoids, were very common in 1960, and the must survive +/-25V had to do with back-EMF from the operations of such solenoids. | |
| May 6, 2021 at 22:10 | comment | added | Raffzahn | @poncho Well, it's should not (not must not). After all, a receiver circuit wobbling in that area between 0 and 1 is a very bad idea.. | |
| May 6, 2021 at 22:06 | comment | added | poncho | "should not be detected as a signal"; rather, need not be reliably detected as a signal. It may be - the standard does not require the receiver to give any guarantee in this case. | |
| May 6, 2021 at 22:00 | comment | added | Raffzahn | @natevw No typo. Two different kinds of illegal. The area below 3V (-3..+3V) is simply not a valid signal, so should not be detected as a signal. The +/-25V regulation is a security margin. While standard conform devices should not apply more than +/- 15V, they must be able to cope with up to +/- 25V without malfunctioning. | |
| May 6, 2021 at 20:45 | comment | added | natevw | "defines +/- 3..15V" — "forbids … below 3V and above 25V" — is this a typo or was the lower range always 3V but the upper range different criteria for 15V max vs. 25V max? | |
| May 6, 2021 at 16:39 | history | edited | Jean-François Fabre | CC BY-SA 4.0 |
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| May 6, 2021 at 14:25 | history | answered | Raffzahn | CC BY-SA 4.0 |