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Current Loop to RS232 Converter

IP.com Disclosure Number: IPCOM000042066D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03

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Peitzer, HS [+details]


Terminal devices such as printer 10, that are configured to communicate with a current loop interface 11, 12, can be adapted to an RS232 interface with the use of circuit 14. For typical current loop interfaces, the light-sensitive transistor 20 of an optical coupling serves as the transmitter. When saturated, current flow is limited by series resistors 13. The light-emitting diode 18 of a second optical coupling serves as the receiver. In this example, diode 18 also monitors data being transmitted. RS232 output 15 is derived from the voltage drop across resistors 13. When current is off, the output will be +5 volts. When current is on, the output will be approximately -11 volts. RS232 input 16 drives PNP transistor 17 into cutoff to break the current loop.