Ryan,

A very thorough treatment of the parallel port interface you will find :
http://www.beyondlogic.org/

If you read the EPP part you will find out that for output only, you can use
the method of address- and data register also with a standard parallel
printer port. So with five (LS)374 (*not 473*) you can have 32 outputs. Some
Googling and you'll find the datasheet.

For low voltages (15V) and low currents (0.5A) a solid state relay will be
overkill. A small (reed) relay will do. Farnell sells HE221A0590 from
Breed/Hamlin priced about 3 euro. FAIK it's one of the cheapest. This relay
will need only a 15mA at 5V so with all outputs on, you still require only
about 0.5A. You'd better not use this relay to switch the mains unless you
want to start fireworks.

The outputs of the 374 cannot source the 15mA required so you have to use a
transistor or an open collector/drain buffer p.e. a HC07. Keep in mind that
a transistor will invert the logic. Always place a diode parallel to the
relay coil.

pieter

"Ryan Kremser" schreef in bericht
s.com...
actually solid state would probably be a better solution for me, for the
most part all i'll be switching is 5 or 12 volt lines. If you could
point me to some more information with the printer port it would help me
out a lot, so far this seems to be the best option. I was actually
planning on running this through dos, complied used c++ as there will be
no monitor on the system and it will require fast boot and response
times. Any better alternatives to dos or will i still be able to access
the parallel port in the same way. again thanks in advance

petrus bitbyter wrote:
Ryan,

You can use an EPP (printer) port. This way you can use a separate
address-
and databyte. Using a 374 type of address register you can enable or
disable
up to eight 473 type data registers without address decoding. That gives
you
256 output pins which sure will be enough.

Do you really need relays to switch your equipment? They tend to be
pretty
expensive and need a good amount of current. So for twentynine relays
you
may end up to need some 3A of current. If you want to switch the mains,
solid state relais are a better choice. (Although they are not cheap
either.) You can also use triacs and optocouplers but you will need some
extra parts so you're building your own solid state relais this way.

Except from the hardware you'll also need some software. Newer (NT
based)
Windows versions will not allow you to write directly to the output
ports.
One of the methods to circumvent these problem is using a special
driver.
For instance:
http://www.paraport.net/

pieter



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