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From: "Spehro Pefhany"
Subject: Re: question on pic circuit
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Mon, 09 Sep 2002 20:45:51 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 16:45:51 EDT
The renowned pjoeter wrote:
> i've found a circuit on
> now, before i'm going to build it, i first want to understand the
> circuit, so i've got a few questions:
> - pin 5 of the first DB9 connected to ground, pin 5 of the second DB9
> connected to 5V (is it save to connect both connectors to a single pc
> at the same time?)
One's for programming one for RS232 comms. You can't use both at once
with normal grounding or you'll have some "interesting" problems (it won't
work and the 78L05 will get hot). If you use a laptop on one of the
ports it should work with both plugged in.
> - for in-circuit programming: P-Voltage (pin 3 of second DB9)
> RS232 signals are -12V to +12V; 1N4742 is a 12V zener (1W), so with
> +12V on pin 3 I don't see probs, but with -12V at pin 3 you have 12V
> directly over diode D3 (signaling diode 1N4148 that's conducting)...
> (you're going to blow up that thing i guess)
An RS-232 circuit could put out something like 500mA and still meet the
standard, but most are limited to <20mA, so it will work in a hobby type
application, most times. Note that this current supplies the 5V by going
through D4 as well. D4 is a potential problem area with the external 5V
supply depending on tolerances. I haven't analyzed what all they are doing
with the PGM DB-9 though.
> - why don't they use a max232 to convert the levels of the first DB9
A max232 costs more than those few cents worth of parts.
> (above). how does that circuit work? (i don't get it... :( )
It borrows the negative supply from the received data to send with.
"it's the network..." "The Journey is the reward"
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