Roger Weichert wrote:
> SUPPLY . Crudely, the boards only use a 5.6v zener to regulate the
> supply to the original micro and two other housekeeping IC's . so I
> assume that needs to be addressed.
>
> The board runs off 12V ac, via a rectifier into a 22000uF cap. Then
> it has a 82 ohm resistor, a zener to drop it to 9 volts . then
> another resistor (120 ohm) and the 5.6 volt zener mentioned above  ..
> crude !! .. but they've been running for 20 + years.

Perhaps you could replace the second resistor and zener with a 7805.  Then
everything would run off the regulated 5V.

> CRYSTAL . the original uses a 4.9152 MHz crystal.
>
> If I kept it in the same spot and perhaps fitted a 4MHz one initially
> (to simplify the timing in the software until I get it going
> properly) . is it likely to cause any problems being 25 - 30 mm from
> the new micro . or should I just plan to fit one on the new board?

Does the micro need to perform accurate timing?  If it's just implementing
some logic, then use a PIC with a internal oscillator.

> The software requirements are pretty basic, so I don't envisiage any
> major problems there . just 4 input links that set 'cycles per
> minute' and the 'output pulse ratio' . another input to monitor any
> 'overcurrent' condition (Hi or Lo) . and four output pins to drive
> some fets (via an interface chip) at 50 - 60 cycles per minute . 6 -
> 12 hours a day, everyday!

This sounds like it's just implementing custom logic where anything within
some minimum response time is good enough?

What does the rest of the board do?  Why does it cost $1200?  Is this the
artifically inflated end user replacement price or include the cost of
sending someone out to do the replacement?  A lot of things are not clear
from your post, but it could possibly be cheaper, and certainly more
reliable, to create a new functionally identical board with a PIC on it than
a daughter board.  In case of failed processor, swap the whole board with
the new one instead of swapping the processor with a daughter board.

Either way, this is the kind of thing we do.  Give me a call if you want to
kick this around.  (978) 742-9014.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist