[cctalk] Re: RS232 Spec

Why 5 mA current loop ? If the spec is silent on the rationale, and I have not checked, I would posit that it is to achieve (opto coupled) galvanic isolation and that 5mA is the classic "typical" current for "lighting" an opto coupler. Galvanic isolation is always a good design feature, but audio equipment has an organic diagnostic feature which encourages good practice ... Martin -----Original Message----- From: Steve Lewis via cctalk [mailto:cctalk@classiccmp.org] Sent: 20 April 2025 11:12 Anyway, just poking around, no specific question here :) Wel, except a related/unrelated question -- why does MIDI use 5mA current loop? And is that why MIDI adopted the joystick port? (IBM original joystick card spec, didn't it involve measuring electrical discharge? so it is effectively a current-loop port? is that somewhat accurate?) -Steve On Sun, Apr 20, 2025 at 12:56 AM Chuck Guzis via cctalk < cctalk(a)classiccmp.org&gt; wrote: