B&B Electronics Parallel Port Input/Output Converter PPIO - Manual (Page 30)

PPIO2899 Manual
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
29
E
XAMPLE
U
SE OF THE
PPIO
Figure 5 shows a simple use of the PPIO to create an automatic
Heating and Air Conditioning system. The top relay controls the Air
Conditioning system. The other relay controls the heating system.
The top thermostat controls the Air and the bottom one controls the
Heat. On the PPIO I/O two and three are outputs to control the
relays. PPIO I/O zero and one are inputs that are controlled by the
thermostats. The way the automatic system should work is that if the
temperature is above 78 degrees the Air should be on. When the
temperature is below 72 degrees the Heat should be on. When the
temperature is between 72 and 78 both the Heat and the Air should
be off.
Each PPIO output can handle up to 500 mA. This means that
the relays have to take less current than that at 12 volts. You also
should not exceed 2.25 watts of dissipation in the PPIO. To measure
this turn on a relay with the PPIO and measure the voltage from the
PPIO I/O pin to your power supply ground. You should get a voltage
around 1.5 volts. If you multiply the voltage you read above by the
current the relay draws you will have the dissipation of that PPIO I/O
pin. For example, if you measure 1.65 volts and your relay draws
100 mA then 1.65*0.1 equals 0.165 watts. If you add up all of the
PPIO loads THAT CAN ALL BE ON AT THE SAME TIME you
should not have more that 2.25 watts. In our example you should not
have both relays on at the same time (you would have the Heat and
Air both on at the same time!) so you only need to worry about the
dissipation of one relay.
Figure 6 shows some GWBASIC code for a simple way to
control the Heat/Air system. Line 100 was explained before. Line
120 forces all the PPIO outputs HIGH (or OFF) so that when you
start the program both relays will be OFF. It is also needed so that
PPIO I/O zero and one can be used as inputs. Lines 160-180 input
the data on all eight PPIO I/O points and puts it in variable IB. Line
190 forces all bits except the first two to ZERO. This leaves only the
status of the two thermostats.
At this point IB can only be equal to zero, one, two, or three. If it
is ZERO then both PPIO I/O inputs are HIGH and both thermostats
are open. This means that the temperature must be below 72
degrees and we want the heat ON - see line 210. The only way IB
can equal one is if PPIO I/O bit zero is ON (LOW) and bit one is
OFF (HIGH). If bit zero is closed the temperature should be above
78 degrees and the other thermostat should be on also. If we get
this condition something must be broken so we turn off both the