Capasitors on DB9 serial port

Moskog

I added a DB9 serial connection port to my project to be able to upload software using my Stache. In the Stache quick start I found how to connect the DB9 to the BS2,· and that works fine.

Later I read my Basic Stamp Syntax and Reference Guide, page 28 and discovered there have to be two capasitors, both 0,1uF connected to pin 4 on the DB9.

I suppose they have to be there but why do I need them?


KjellO

allanlane5

They don't HAVE to be there. Their purpose is to 'decouple' the DTR line from holding your BS2 in 'reset'.

You see, when using the IDE, when you click "program" your PC uses the DTR line as a 'master reset' to reset your BS2 and prepare it for the programming sequence. Well, some 'stupid' programs (like Hyperterminal) hold the DTR line high ALL the time. To let you use Hyperterm, the BOE-board had these two capacitors, so when Hyperterm raises DTR, all the BS2 sees is a "Blip" (which STILL resets it, briefly) which doesn't HOLD the BS2 in RESET.

So if you're not using Hyperterm on the PC to monitor your BS2, you don't need those two capacitors.

stamptrol

They have been added to improve performance of the auto reset of the processor after downloading. They help prevent random resets as the programming cable is plugged in and out.

If your system is working as is, don't worry about it. Many year's worth of boards did not have the capacitors.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Moskog

Well, thank you allanlane5 and stamptrol, I quess I never will use the Hyperterm and I don't think a random reset will hurt much.
Thank you again for your explanations.

KjellO