Two boards, two power supplies

metron9

I have a BS2 Homework Board powered by a 9volt battery
Another developement board for another PDIP using a 9 volt wall plug in. The board has a regulated +5V

I want to send high low signals from the stamp to the other board.

I think I should tie the grounds together, and then hook BS2 board P0 (output mode) to an input pin on the other board and read the high low from that board?
Also set BS2 P1 to input mode and hook to other board output pin?

But I have this nagging question what happens if I connect both grounds and both +5 volts on both boards? Like 2 batterys in parallel?

I have to laugh at myself for being so dumb about some things while working on the complex project at the same time.

Beau Schwabe

metron9,
You have the right idea, connect both grounds.· I would suggest placing a current limiting resistor of 1K or so inline just in case
both stamp pins happen to be outputs.

Stamp I/O #1 >---/\/\------------------//------------------/\/\---< Stamp I/O #2
Stamp GND #1 >-------------------------//-------------------------< Stamp GND #2

You said...
I have this nagging question what happens if I connect both grounds and both +5 volts on both boards? Like 2 batterys in parallel?

You really don't want to do this.· Comparing this to two batteries in parallel, if one battery is slightly off (not the same voltage),
then BOTH batteries will fight one another.· Consider the schematic below...

                                    R_batt
          o----[noparse][[/noparse]+ 5.8v Battery -]----/\/\----o
          |                                  |
<---------o                                  o---------> 
          |                         R_batt   |
          o----[noparse][[/noparse]+ 6.1v Battery -]----/\/\----o

Assume that R_batt is the battery's own internal resistance of about·2 Ohms
·
Total_R_batt =·4 Ohms
·
...Because the batteries are slightly different, there will effectively be about 0.3 Volts across the Total_R_batt.
This equates to 75mA ( I = V / R ) that BOTH batteries have to dissipate.· That's 446 milliWatts·( P = I * V ) of unnecessary heat.
·
The same thing can happen from a voltage regulator.
·
















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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

KenM

Tie the grounds together.

Do not tie the +5 volt supplies together.

And for the output of one stamp pin to the input of the other stamp pin, connect a 220 ohm resistor. Doing so should protect both stamps from a programming error that results in both stamp pins outputs with one high and the other low.



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Ken

metron9

Thank's that answers it then. Now am going to try and figure out the final step and that's getting a seconds elapsed timer going, It's not two stamps though I am using an ATmega48 from ATMEL.com and programming it with bascom-AVR, I just spent the last 8 hours reading the 300 pages of spec sheet on the internal ocoloators the 16 bit counters and such but still no clue on how to get a clock signal into my program on that side. I will have to search again on these boards and experiment with timers on the BS2 I think. So my brain is totally full tonight and that means I need to get some REM sleep so it can iron out all the details. It sure is funny sometimes I go to sleep without a clue to the problem i was working on all day and then bingo one cup of coffee and problem solved.