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Circuits > PC related > Parallel Port Relay Interface
Parallel Port Relay Interface
Below are three examples of controlling a relay from the PC's parallel
printer port (LPT1 or LPT2). Figure A shows a solid state relay controlled by
one of the parallel port data lines (D0-D7) using a 300 ohm resistor and 5 volt
power source. The solid state relay will energize when a "0" is written to the
data line. Figure B and C show mechanical relays controlled by two transistors.
The relay in figure B is energized when a "1" is written to the data line and
the relay in figure C is energized by writing a "0" to the line. In each of the
three circuits, a common connection is made from the negative side of the power
supply to one of the port ground pins (18-25).
There are three possible base addresses for the parallel port You may need to
try all three base addresses to determine the correct address for the port you
are using but LPT1 is usually at Hex 0378. The QBasic "OUT" command can be used
to send data to the port. OUT, &H0378,0 sets D0-D7 low and OUT, &H378,255 sets
D0-D7 high. The parallel port also provides four control lines (C0,C1,C2,C3)
that can be set high or low by writing data to the base address+2 so if the base
address is Hex 0378 then the address of the control latch would be Hex 037A.
Note that three of the control bits are inverted so writing a "0" to the control
latch will set C0,C1,C3 high and C2 low.
Parallel Port Relay Interface circuit
Title: Parallel Port Relay Interface
Source: unknown
Published on: 2007-07-21
Reads: 1619
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