A lot of serial equipment back then still had 20mA current loop. For example, the ASR 33 Teletype[1], which was often connected to PDPs of its era did not have

A lot of serial equipment back then still had 20mA current loop. For example, the ASR 33 Teletype[1], which was often connected to PDPs of its era did not have RS-232. [1] https://en.wikipedia.org/wiki/Teletype_Model_33#Communications_Interface https://en.wikipedia.org/wiki/Teletype_Model_33#Communicatio...