Radio Shack Quick Printer II Reference Handbook

Memory Mapped
External Device
Figure 20 shows a logic diagram of a memory-
mapped coffee pot switch (of course it could be
any other control switching circuit). It was
drawn to illustrate a technique and should not
be considered a final working design.
The following software requirements were
assumed:
1. Needs to control a relay that will control the
power going to the coffee pot.
2. Memory
mapped at Hex address SFFF.
3. Design should include some method to test if
relay
is on or off.
4. A program
to control this external device is
shown below.
At line 600 in the program, the CPU will POKE
data 2 (Hex (2) into memory-location -4092
(Hex 8FFF). In binary, 8FFF is 10001111
11111111.
This 16-bit word needs to be decoded. Z1, Z2A
and Z3 decode the address lines, producing
8FFF* at the output of Z3. This signal is
applied to the input of Z2 C and D. The POKE
statement will cause WR* to go low at the same
time as SFFF* goes low. Therefore the output
at Z2 C will go high. This signal is called the
WRITE and is applied to Z6, a latch. 02 in Hex
is 0000 0010 in binary. Therefore, at the same
time that WRITE goes high, Data bus bit 01 will
be high. The high at D1 will be latched or stored
in Z6. The resulting high at the Q output of Z6
will be passed
thru relay driver Z7, and the relay
will close
and stay closed. One set of K1 's con-
tacts is used to turn on the coffee pot. The other
contact is used in an acknowledge feature.
At line S0(/) of the program, the coffee pot cir-
cuit is once again accessed. Address decoding
1210 REM ***COFFEE POT POWER CONTROL***
22121 CLS
PR I NT @512, "DO YOU WANT THE COFFEE POT TURNED ON";
32121 INPUT A$: IF A$ = "NO" GOTO 78421
42121 REM ***IF NO, BRANCH TO "RUNNING LATE" ROUTINE***
52121 REM ***ANYTHING ELSE, TURN ON COFFEE POT RELAY***
600 POKE - 4092,2
700 REM ***NOW TEST IF CONTROL RELAY CHANGED STATES***
8021 B=2: A=PEEK (-4096)
92121 IF A AND B = 21 THEN GOTO 19821 ELSE GOTO 3744
1212121 REM ***IF RELAY WORKED, BRANCH TO "WEATHER SENSOR" ROUTINE***
1100 REM ***IF RELAY DID NOT WORK, BRANCH TO "SYSTEM FAULT ISOLATE" ***
86

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English