31 Jan
2025
31 Jan
'25
4:58 a.m.
On Fri, Jan 31, 2025 at 04:56:06AM -0700, ben via cctalk wrote:
[...]
I always wondered why one needed a 25 pin connector?
Now every thing seems to be just 3 wire TTL.
Before RS232, how many wires where needed for the current loop
and did they have standard connector?
I can see 2 wire pairs, and ground.
The minimal three-wire variant carries *only* payload data, and does not
have flow control or other out-of-band control signalling. Relatively
complex devices can run a protocol which multiplexes everything to reduce
wire count, but this complexity is not always warranted when running extra
wires is cheap.
There are a half-dozen out-of-band control signals. That makes nine wires,
which conveniently fits in the now de facto DE-9 serial port standard.
However, some devices are complex enough to want to out-of-band control
*data* (e.g. to send a phone number to an autodialler) and so there is an
optional secondary channel. This adds "only" five more wires, bringing the
total to 14, and so theoretically one could use a DA-15. However, there were
other obscure bits of signalling that some devices wanted, and the next
standard size is DB-25.
It's not always all-or-nothing either: for example, late-model dialup modems
use "AT" commands to multiplex the control data stream and some control
signals onto the payload data, but still use RTS/CTS for flow control.