Al Kossow wrote: The short-haul modems I used in the past (not Black Box, Madzar, iirc) were designed for 4-wire leased lines, and had a limit of a couple of miles. They expected a short-haul modem at each end (worked in pairs). I don't think they were designed as current-loop converters, so the current may not be at any standard setting. They may not even be current-loop, basically just opto-isolated op-amps on twisted pairs. Lightning *frequently* killed them, but rarely made it to the equipment they were attached to. Open them up, and check for "burn" holes (charred spots) in the top of the IC's. iirc they only had 2 IC's, a transmitter and a receiver. They maxed out about 9600 baud, and you had to run them slower the longer the wire between the units were. They were designed to be faster than modems (which were at 300 baud when we started using them, 2400 baud when we quit).