Been many years since I installed mechanical teleprinters but I don't remember a 4ma condition. Start pulse was an open? On 3 December 2014 at 10:59, wrote: > > Your point about an open line at high impedence makes sense. > > > > However, as I understand it, current loop originated with teletypes. I > was > > under the impression that the whole thing was electromechanical and the > > circuit was open when the 'switch' was open. Did they have a resistor i= n > > parallel with the switching element, or something like that? > > > > ~ Bob Ammerman > > RAm Systems > > That is my understanding as well, and matches with how long distance > telegraph lines used to operate, and all microprocessor development syste= ms > that I have come across worked the same way as well. > > The only time I have seen 4mA and 20mA used is analogue instrumentation > loops where the signal varies 4-20mA. > > > -- > Scanned by iCritical. > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .