On 2010-01-03, Karl Townsend wrote:
My winter project is a microcontroller to ventilate and water my
greenhouse/high tunnel. Parallax has a sale on controllers with free
shipping through tomorrow. I plan on using Opto 22 boards and IO blocks for
this project. Parallax makes a module just for this:
http://www.parallax.com/StoreSearchR...9/Default.aspx

(parallax part number 27945 if link above breaks)

I did a count this morning and this is no where near enough IO. Parallax
makes a 40 pin stamp with 32 IO that would work. But they don't make the
above module for it.

Well ... the Opto 22 boards which I have have a maximum of 24
I/O channels -- in part because it is fed through a 50-pin cable
(power ground, +5V, and a signal-ground pair with each channel, so a
long cable will have a ground between any two signal lines to reduce
the chance of noise coupling from one line to another.

You would need two 50-pin ribbon cables to handle the 32 I/Os
which you want, and that would typically go to two Opto 22 boards so you
have easy access to the screw terminals for each I/O channel.

I don't want to paint myself into a corner. Should I
look at building a custom 27945 module for the 40 pin stamp? (I don't
normally do this kind of work) Or is it no big deal to use two stamps and
link them to communicate? Other ideas?

I would go for the latter approach. This has the advantage of
reducing the load on any one stamp. You simply have to define some kind
of I/O protocol for the communication between two stamps and make one
the master and the other the slave. Or -- perhaps -- make one handle
all outputs and one handle all inputs for monitoring results.

Good Luck,
DoN.

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