I have done something similar for the monitoring of push buttons then
controlling the output relays/leds to indicate the state of the input
circuits.

I used a combination of 74HC595 serial latches controlled by the 16F84.  As
each of these requires a data and clock line these can all be paralled and a
separate enable line is required for each latch, therefore for 32 outputs
there would be a requirement for 6 control lines from the 16F84.

This would leave 6 for input control/sensing.

As an alternative I have also used the I2C bus to control output devices (8
outputs) PCF8574 if I remember correctly.  Each of the devices has 4
addresses allowing 32 outputs from 2 control wires, in addition there is an
"A" part that allows an addition 4 addresses giving the possibility of 64
outputs from  the 2 wire bus.

Hope this helps

Rex Pearson
NT Police, Fire & Emergency Services
----- Original Message -----
From: "Greg Peyton" <GPEYCO@CS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 26 June 2000 05:09
Subject: Relay question...


> I've been charged with building an improved test fixture (T/F) at work and
> would appreciate some help from the group. Before I begin I'd like you to
> know that I'm not an engineer, only a technician who has been studying the
> pics that have been used in our products for the last few years.
>
> In the past, we've used manual rotary switches as a signal path through
our
> T/Fs. This is an inherited design, one which I've never been happy with.
If
> it were up to me we would have scrapped this concept many, many years ago.
> When new the T/Fs work fine. With time these contacts wear and tarnish,
> eventually cause false readings, resulting in final test product rejects.
> Eventually the outer coating on the contacts would wear off, revealing
only
> brass. The contacts became erratic and drove up labor costs for testing.
The
> product is a sensitive instrument used for testing of telephone lines and
> therefore requires very low, or at least resonably matched low resistances
in
> all contact points.  Changing over to solid silver alloy wafers contacts
and
> a lubricant greatly improved T/F lifespan but at a large $ cost. The T/Fs
> themselves are entirely manual, time consuming to use, build and maintain.
>
> I've come to the conclusion, and I invite different optinions, that we'd
be
> better off with an array of relays under direction of a ucontroller.
> The T/F presents to the product a variety of resistances, capacitances, ac
> and dc voltages (up to 300V) to be measured in the different settings.
Simple
> stuff but nonetheless we require a degree of accuracy that is repeatable.
>
> My question (and I do have one) is this: What is the best way of designing
> and controlling an array of latching relays?
>
> We use NEC EB2-5S 5VDC latching relays in our product so I have an huge
> supply to work with. The coil of these relays is stated at 250 ohms. I
would
> like to have available about 32 of these relays but a design that would
allow
> expandability for future unimagined tests. One of the engineers has
proposed
> that I use a 74HC373 off of the pic ports to activate the relays. His
> thinking is that using two of the '373 output pins for each relay, in a
daisy
> chain, I could energize the desired relay for a few miliseconds. I
understand
> this, having seen it used in one of our products, and believe that it is a
> possible solution for this T/F business. I have huge respect for our
> engineers but something in the back of my head tells me that there is a
> better, more efficient or possibly better organized way of doing this.
> At this point I can see that there are several competing factors involved;
> programming efficiency, component count, simplicity for troubleshooting,
> allowance for future expansion, etc.
> Obviously this isn't the ultimate solution to the problem but I know that
> we're not going to be able to convince the guy who controls the purse
strings
> to buy new computers, interface cards, dataloggers, calibrators, etc. To
> improve the existing setup is what I'm trying to accomplish. Besides,
> anything I learn from this helps me out.
>
> Does anyone have experience with this kind of thing? I'd be willing to
> entertain other ideas since I haven't bought any parts yet.
> Any web sites or application notes that deal with this type of thing? I
don't
> believe in reinventing the wheel. Every time I do that I get admonished by
> the boss for wasting time (three times before getting it right is normal
for
> me).
> I'm looking for a dialog on this so feel free to respond with questions
and
> advice before flames.
>
>
> -Greg Peyton
>