3 ms·

Is there a word to describe the +/- signal behavior of rs232? It's like a differential signal, but with a pesky common ground, bother.

robinsonb5 4y ago
I'd probably go with "anachronistic" - RS232 predates TTL logic! Interesting that the bit-level protocol is still very widely used today, even if the signalling is now more likely to be 5v or 3v3 TTL signalling bridged to USB.
- masswerk 4y ago
  It's up to ±25V, a voltage swing of 50V. (I believe, ±12V was common for TTYs.)
ajross 4y ago
My understanding is that it was for driver circuit compatibility with older current-loop TTY devices. Early Teletype et. al. terminals were designed to run over potentially long wires with high resistive losses[1], and to be received by 1960's electronics with very low impedances (the ancestral devices were mechanical switches!). So the idea was you'd just switch a big DC supply onto the lines and figure it would be enough to get 20 mA at the other end, so that it could see it. When RS-232 came along, they deliberately specified a very wide band of allowed voltages so that in many cases manufacturers could just cable the current loop device in parallel with a resistor and be compliant. Don't quote me though, I don't remember where I read this. [1] So... yeah, it's basically solving the same problem as differential signaling but at a different scale.
kevin_thibedeau 4y ago
Bipolar NRZ.
Animats 4y ago
It's not differential, just polarized. -12V or +12V originally. Extracting signal polarity is more reliable than triggering on a threshold, or it was before electronics got good. It's a telco thing. Polarized signaling goes back to the original Edison stock ticker. Polar relays were once used on Teletype circuits.
fuzzfactor 4y ago
Low voltages near zero on any pin either + or - side are ignored, below a certain threshold of absolute voltage either way it's no-man's land. Signals are only considered real if they are boldly above the threshold, preferably close to +/- 12VDC either way to reliably qualify as a space or a mark, but no higher amplitude than +/- 25VDC. This helps to compensate for unavoidable differences in absolute ground potential between remote locations. The data signal is basically just square waves since it's ones & zeros, but hook a speaker up properly and that's the unforgotten audio sound of dial-up internet.

robinsonb5 4y ago

I'd probably go with "anachronistic" - RS232 predates TTL logic! Interesting that the bit-level protocol is still very widely used today, even if the signalling is now more likely to be 5v or 3v3 TTL signalling bridged to USB.

masswerk 4y ago
It's up to ±25V, a voltage swing of 50V. (I believe, ±12V was common for TTYs.)

ajross 4y ago

My understanding is that it was for driver circuit compatibility with older current-loop TTY devices. Early Teletype et. al. terminals were designed to run over potentially long wires with high resistive losses[1], and to be received by 1960's electronics with very low impedances (the ancestral devices were mechanical switches!). So the idea was you'd just switch a big DC supply onto the lines and figure it would be enough to get 20 mA at the other end, so that it could see it. When RS-232 came along, they deliberately specified a very wide band of allowed voltages so that in many cases manufacturers could just cable the current loop device in parallel with a resistor and be compliant. Don't quote me though, I don't remember where I read this. [1] So... yeah, it's basically solving the same problem as differential signaling but at a different scale.

kevin_thibedeau 4y ago

Bipolar NRZ.

Animats 4y ago

It's not differential, just polarized. -12V or +12V originally. Extracting signal polarity is more reliable than triggering on a threshold, or it was before electronics got good. It's a telco thing. Polarized signaling goes back to the original Edison stock ticker. Polar relays were once used on Teletype circuits.

fuzzfactor 4y ago

Low voltages near zero on any pin either + or - side are ignored, below a certain threshold of absolute voltage either way it's no-man's land. Signals are only considered real if they are boldly above the threshold, preferably close to +/- 12VDC either way to reliably qualify as a space or a mark, but no higher amplitude than +/- 25VDC. This helps to compensate for unavoidable differences in absolute ground potential between remote locations. The data signal is basically just square waves since it's ones & zeros, but hook a speaker up properly and that's the unforgotten audio sound of dial-up internet.