ZX Computings - OpenLibra

and 2.5 volts. This will produce
a number between 0 and 255
on PEEKing at the INPUT from
the port. There are + 5 volts
and 0 volts connections from
the pack, so these can be used
to create the voltage required.
These should be used with care
as they may cause the crash of
the computer if too much current
is taken.
The output of the Digital to
Analogue converter will convert
the number between 0 and
255 into a voltage between 0
and 2.55 volts. This can be adjusted
within the port, if it is not
correct, and a complete circuit
diagram is given. The accuracy
is ± 5% on both the analogue
input and output.
The resistor could be
measured using this device by
connecting them as shown in
Fig. 1. By knowing the voltage
across the known resistor and
the voltage output by the
analogue port, the resistance
can be calculated. As the
resistance of the unknown
resistance rises, the measured
voltage across the known
resistor will fall. The computer
can then be used to work out
the unknown resistance.
The C Pack
The 'C' pack again requires the
use of a *P' pack or another
port. The 'C pack can plug
directly onto the connectors at
the back of the 'P'.
The 'C' pack provides eight
relays which can be controlled
by the output of the port. The
relays are turned on by making
any of the eight bits Binary 0.
The outputs from the back are
by the same 2 mm sockets as
used on the 'A' pack. The outputs
consist of eight single contact
switches, which connect
to a common wire. They are not
connected to the computer at
all. The contacts are normally
closed and can carry 12 volts at
1 amp maximum. Therefore
when the computer is switched
on and the output from the port
is 255, all the switches are
closed. I would have thought
this would be a disadvantage in
a controlled situation, as all the
devices would be turned on
under no control from the computer.
The inputs consist of a set of
eight resistors connecting each
of the input connectors to 0
volts. This means that when
PEEKing the port with nothing
connected the user will see 0.
There is no buffering between
these input pins and the input to
the port, so that the application
of any voltage above + 5 volts
could not only damage the 'C'
Port 'P' pack
pack, but also burn out the 'P'
pack as well.
These devices are intended for
use in educational or control
purposes. Games could be
another possible use, but the
price of these packs might put
that out of bounds. The limit of
only 5K of RAM and only one
port might prove restrictive in
use, but up to 128 different
1 bit devices can be controlled.
The boxes are robust and
could be used in schools without
needing a set of expensive
plugs and sockets. I have
doubts, however, over the inputs
to the 'C' pack. If any
voltage over + 5 volts is used,
some form of protection will be
required to stop the port blowing
up.
The documentation that
comes with the port is very
clear and simple to understand.
The leaflet included with each
pack details its uses and gives a
program to demonstrate its
capabilities. The current drawn
by these packs is shown in table
1. DCP packs cost £37.95 for
the price of the 'P' pack,
£29.95 for the 'A' pack and
£1 9.95 for the 'C' pack. Both
'A' and 'C' packs must be used
with a 'P' pack or other ports.
Table 1.
45 ma.
'A' pack 80 ma.
'C' pack 775 ma. (Max.)
Solving Equations
This clever routine, written by
Jeremy Ruston, uses Newton's
method for solving equations.
Enter the equation you want
solved for X when prompted by
line 10, then — in reply to the
prompt from line 40 — enter a
starting position for the ZX81
to work from. This should be
M"
either an answer somewhere
near what you believe the correct
answer to be, or — if there
is more than one correct
answer — a number near the
answer you are seeking. Then
press NEWLINE and sit back
and watch the fun as the computer
verges towards the
answer.
To try it out, enter X*X-5
(to find the square root of five)
or X*'3-27.6 to find the
cube root of 27.6.
L- RTF-.N WEWTONS METHOD- FOR
SOL^XHG EOLTRT TON 5
REM BY UEREMY RUSTON
PRINT "ENTER P FUNCTION ";
20 INPUT F S
30 PRINT F$
4-0 PR IMT "ENTER R STRRTIN6 POX
50 INPUT S
60 PRINT 3
7"0 PRINT '* INPUT MAXIMUM ERROR
©0 INPUT ERR
90 PRINT ERR
1O0 PRINT RT 10,10;3
110 LET X=3
120 IF RB5 VURL L'F$.» > -: ERR THEN
3"! OP
130 LET T=URL LF$>
140 LET X =X +0- 00001
150 LET B = (URL TF$> -T> Y0.00001
163 LET 5 = 3-T.'B
1*7® GOTO 100
ZX COMPUTING SUMMER 1982 69