apm279l@vaxc.cc.monash.edu.au (10/26/90)
Hi,
I recently posted here in regard to a problem with poor transmission
from my 28S to a PC via HPREAD. With help, the problem was fixed, so
I thought I'd share the solution;
1. Firstly, I got requests for how to connect a HP28 to a PC. To find
out, simply get a copy of HPREAD.C from an HP28 site (eg
gmuvax2.gmu.edu = 129.174.1.8) - the actual connection is simple
if you have at leas an 8 MHz machine.
2. For some reason (best known to the designers of the PC), there is
no 5 volt output line on the parallel printer port, so you have to
provide your own supply to your detector. One way (as described in
HPREAD) is to tap the printer's own Vcc line; a simple task, but
I wanted to be able to use my detector without a printer nearby.
What I did was to use my PC's 5 volt rail by getting a plastic 4-pin
connector like the one on the top of your floppy drive (with the four
wires to it: red=5V, Yellow=12V, black1=black2=ground). Many PC's
have a spare connector like this hanging loose inside the case for
future expansion. To use my detector, I either use the spare connector
or disconnect one of the drives, and plug in to its supply.
OK: that's fine, (in fact it's a very convenient solution). The problem
is that the supply is a *switching* supply, and it turns out that the
detector's load is too small. The solution is simple; increase the load
by reducing the resistance of the detector circuit. You can do this by
connecting a 10 ohm, 5 watt resistor across the supply (in parallel
with the detector). (In my case, I couldn't find a 10 ohm high power
resistor handy, so I used two 24 ohm resistors in parallel).
WARNING: THE RESISTOR IS INCLUDED SPECIFICALLY TO "WASTE" POWER.
It therefore will get quite hot, so set it in a place
where it is not in contact with plastic (etc). Also, DO
NOT USE A "NORMAL" RESISTOR - it will burn out! - be
sure to use at least a 5 watt rating.
Don't be put off by the warning - I'm very happy with my detector
now, and I have a convenient power supply which doesn't require
two pieces of equipment (the printer as well as the PC).
+5V rail DISK DRIVE
DETECTOR +--------------------------+ SUPPLY
----------- | | ----
| | | | | |
| Pin 2--+----+ PRINTER PORT +---+--------+--+-- red (5V)
| (Vcc) | ------------ \ | |
| | | | 10 / +----+--+-- black (0V)
| Pin 1--+-------------+--Pin 15 | ohm \ | | |
| (Vout) | | (ERROR) | 5 / | | +-- black (0V)
| | | | watt \ | | |
| Pin 3--+----+--------+--Pin 18 | / | | +-- yellow (12V)
| (GND) | | | (GND) | +-------+ | |
| | | | | | ----
--------+-- | ------------ |
| | |
detector | | |
casing +------+--------------------------+---+ 0V
|
--+--
---
-
Cheers,
Peter