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.
