Timeline for Why was RS-232 12V?
Current License: CC BY-SA 4.0
11 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| May 7, 2021 at 18:24 | answer | added | Justme | timeline score: 11 | |
| May 7, 2021 at 17:04 | comment | added | rackandboneman | @Justme yes, the MAX232 and variants - which look like an amazing non-cost-cutting measure :) | |
| May 7, 2021 at 7:57 | comment | added | Justme | @rackandboneman Only receiver must interpret down to +/- 3V properly, the transmitter must still be able to transmit at least +/- 5V into a receiver of 3kohm load. Later equipment did not use separate supplies but used single tranceiver chip with built-in charge pump to generate the necessary +/- voltages from single 5V or 3.3V supply. | |
| May 7, 2021 at 4:29 | comment | added | Jasen | 0v and +2V would work in practice, the MC1489 was designed that way (Jim Thompson the designer said as much in a usenet post), but in the presence of noise having some headroom was a good thing. | |
| May 7, 2021 at 0:20 | comment | added | rackandboneman | If "min +/- 3 volts" was reliably applicable in practice, no 1980s and later small computer maker would have bothered with all the added complexity to provide +/-9V and higher levels .... but most of them did :) | |
| May 6, 2021 at 21:37 | history | became hot network question | |||
| May 6, 2021 at 14:25 | answer | added | Raffzahn | timeline score: 40 | |
| May 6, 2021 at 13:52 | comment | added | tofro | Nitpicking: RS-232 doesn't say +/-12V. It in fact says max.: +/-15Volts, min: +-3V. +/-12V is just a convenient in-between that can be assumed to be available in a computer environment and also provides ample elbow room for STN (early memories used 12V as well) | |
| May 6, 2021 at 13:51 | comment | added | Raffzahn | The 'strong' reason to use 5V is simply to save cost. Why adding the need for a 12V PSU when short distance, like to a printer, works quite fine at 5V as well? | |
| May 6, 2021 at 13:46 | comment | added | davidbak | Total wild guess: It came out of the telecom industry which used dry/wet-cell style battery backup for continuous operation ... 12v was easy to come by? I'll be interested to know the actual reason. | |
| May 6, 2021 at 13:37 | history | asked | Maury Markowitz | CC BY-SA 4.0 |