current loop for modem

elektor july august 1985 A modem, such as the direct-coupled modem featured in the October 1984 issue of E/ektor, opens a whole new world to the computer user by making possible communication between two computers anywhere in the world (provided, of course, they can be coupled to a telephone line). Ironi- cally, although the distance between the computers may be very large, that between computer and modem is strictly limited. This is because the RS 232 input is voltage driven and is, therefore, very susceptible to noise. This is not a new problem: it existed many years ago when, for instance, two telex machines had to be inter- connected. The solution then found, and still in use today, is the current loop. Such a current loop can also be used when the distance between the modem and the computer is relatively large: up to 1 km. A current loop so used converts RS 232 compatible voltages into RS 232 compatible currents. The stan- dard in the RS 232 protocol is a cur- rent loop of 20...