Specifications

BASIC Stamp II

Parallax, Inc. • BASIC Stamp Programming Manual 1.8 • Page 305

Reverse

REVERSE

Reverse the data direction of the specified pin.

•

is a variable/constant (0–15) that specifies the I/O pin to use.

This pin will be placed into the opposite of its current input/

output mode by inverting the corresponding bit of the DIRS

Explanation

Reverse is convenient way to switch the I/O direction of a pin. If the

pin is an input and you Reverse it, it becomes an output; if it’s an out-

put, Reverse makes it an input.

Remember that “input” really has two meanings: (1) Setting a pin to

input makes it possible to check the state (1 or 0) of external circuitry

connected to that pin. The state is in the corresponding bit of the INS

(corresponding bit of OUTS). The demo program below illustrates this

second fact with a two-tone LED blinker.

Demo Program

Connect the circuit of figure I-16 to pin 0 and run the program below.

The LED will alternate between two states, dim and bright. What’s

happening is that the Reverse instruction is toggling pin 0 between

input and output states. When pin 0 is an input, current flows through

R1, through the LED, through R2 to ground. Pin 0 is effectively discon-

nected and doesn’t play a part in the circuit. The total resistance en-

countered by current flowing through the LED is R1 + R2 = 440Ω. When

pin 0 is Reversed to output, current flows through R1, through the

LED, and into pin 0 to ground (because of the 0 written to OUT0). The

total resistance encountered by current flowing through the LED is R1,

220Ω. With only half the resistance, the LED glows brighter.

OUT0 = 0 ' Put a low in the pin 0 output driver.

again:

pause 200 ' Brief (1/5th second) pause.

REVERSE 0 ' Invert pin 0 I/O direction.

goto again ' Repeat forever.