I've been thinking about how things get connected between PC's and PIC's
lately and overall what I can see is:
USB: Nice! But not the lowest possible cost and not supported on some old
PC's Also, requires some driver development on the PC side... not hard, but
still.

RS232: Nice... But you can run out of ports, some current laptops don't even
have serial ports (!) and you have to do all the darn configuration on the
PC to make sure all the settings are right. Has anyone been able to get a
USB to RS232 adapter to work right? I can never get the PC application to
see the port.

Parallel: Sigh... NT and up driver problems (IF you don't simulate a printer
with the PIC) but there are solutions for that. One the one port usually and
I've seen laptops without parallel ports. USB to parallel adapters are
really shaky... has anyone used one to talk to anything other than a
printer? The two I've tried don't show up as LPT ports on the PC and so old
applications don't see them.

Of all the above, probably the most reliable, simple, and cheapest for us
little guys is the parallel port (this is IMHO and I'm sure the point can be
argued 20 ways from Sunday).

So, if I want to put out something that people can setup from a PC, a
temporary connection via the parallel port is probably best. Now the
question is how do I keep people from bread boarding something that connects
via my device that will blow up the PC (as well as me probably) when they
screw up?

I went on a hunt for parallel port isolation and, not being the most
brilliant analog electronics engineer (ok, so I'm a complete moron in that
area) I really don't seem to understand most of what I found.

I understand that the port can sink 24mA and only source 2.6mA so obviously
its a good idea to drive your loads from the port, through the load, then to
+5v rather than ground.

Safe current limiting can be had with a series resistor of 1.6k or more
right? (4.2V / 2.6mA) But that doesn't leave much drive... so you can add a
driver, but wouldn't it be better to prevent the pin from sourcing anything
by putting a diode long with the resistor in series and then reducing the
resistor to 175 or more ohms (4.2V / 24mA)?


                       +5
                        |
                        /
                        \
                        /
       220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>
 _|_/
//_\ 5.1V
  |
 GND

A pull-up resistor is added to the output to drive TTL or other loads and a
Zener could be added to ground to prevent over voltage or driving the pin in
reverse. That seems to me to be a nice little, very low cost, easy to work
with, circuit to provide some basic protection for kids or students to tack
things onto.

Not wanting to assume I have a clue, I searched the internet for other
solutions. Tomi Engdahl says:

"The following circuit uses two 1N4148 diodes to protect parallel port
against higher than +5V signals and also against wrong polarity signals
(power on the circuit is accidentally at wrong polarity. "

          Diode
          1N4148  4.7K
parallel  >-|>|-+--\/\/\/--etc...
port data       |
pin             +-|<|-+
            1N4148    |
parallel  >-----------+
port ground           |
                    Ground

"Adding even more safety idea: Replace the 1N4148 diode connected to ground
with 5.1V Zener diode. That diode will then protect against over-voltage
spikes and negative voltage at the same time. "

Ok, he has his series diode in the exact opposite direction. Could somebody
please tell me if I'm just being stupid? Other than that (and the much
larger series resistor) the circuit is the same.

I also found on piclist.com:

"Another way to protect against over-voltage is to put a regular diode with
the cathode on the signal line and the anode on the power supply line. Keep
in mind that this will cause the power supply to receive the over-voltage
and may result in damage to other components on the board."

Standard diodes are cheaper... any reason why I can't put ONE Zener in the
power supply and use regular diodes on the 8 pins?

x8:
                       +5
                        |
  +5                    /
 _|_                    \
_\ /_                   /
  |    220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>

x1:

  +5
 _|_/
//_\ 5.1V
  |
 GND

---
James Newton, webhost piclist.com (former Admin #3)
jamesnewton@piclist.com 1-619-652-0593 fax:1-208-279-8767
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