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From: "Michael F. Coyle"
Subject: Re: RS232 newbie - in need of assistance!
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 03 Jan 2003 20:23:47 GMT
NNTP-Posting-Date: Fri, 03 Jan 2003 15:23:47 EST
Organization: Optimum Online
"Matt Foster" wrote in message
> Here's my first attemot at a schematic - please ruthlessly rip it to
> laugh at my incompetence and then give me a better one :-D
It's pretty good but I would suggest a few changes:
1. The switches should not be in series with the resistors -- when a switch
is open, the input pin does not go down to zero volts. Instead it floats
around between 0 and 5V because MOS inputs are high impedance.
Instead, connect each input to a resistor to 5V and a switch to ground.
When the switch is open the resistor pulls the input to 5V; when the switch
is closed, the pin is grounded (and as a side effect, current flows through
the resistor to ground, which needn't concern us here).
A further simplification is to eliminate the resistors altogether! (Just
have switches to ground.) Port B on PICs have internal pullup resistors
that can be turned on within the program. This saves you the five
2. If you havent bought the 16F84 yet, consider using the PIC16F628 instead.
It's a newer part, has more memory, and has a built-in UART which makes
programming this application a snap. You would have to do your serial
transmission in software on the 84.
If you go for the 628, you may have to use a different crystal frequency to
make the built-in baud rate generator to work correctly. Most standard baud
rates are unrealizable with a 4 MHz oscillator. A better value to use is
3.6864 MHz. Then all standard baud rates from 300 to 230,400 are available.
See the 628 data sheet for UART and baud rate generator details.
3. Check your MAX232 data sheet -- mine shows 1uF capacitors, not 10uF. (I
think some older data sheets used 10uF.)
4. I haven't checked your serial port pinouts -- you're on your own there :)
But you're definitely on the right track.
Hope this helps. Good Luck!
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