>1) I plan to use the PIC18F1220's internal 8MHz oscillator and to
>   bit bang a PIC I/O pin for the UART TX part (just because it's
>   more fun to do so and because I won't have to cope with the
>   1220's built-in UART's baud rate divisor limitations).


I don't know what limitations you mean, but unless you specifically need to
use the RX pin for something else, I would use the hardware uart. You do not
say what language you are programming in, but if using the Microchip C18
compiler, there is a software uart module already there, as well as a
hardware uart module. I would also look at having two way communication, and
build in some form of "speed selection" that changes the analogue sampling
rate, by sending commands from the PC. You may also want to have multiple
channels that you can switch off/on, again from the PC. The internal
oscillator should be perfectly adequate for baud rate generation given that
the other end should be crystal controlled in the PC, so you can use pretty
well all your frequency tolerance at the PIC end.

>2) Use a full MAX232 or some diode/transistor-based hack? Consider
>   that communication is only one-way (from circuit to PC COM port).

definitely use one of the MAX series chips, or one of the many clones. You
will thank yourself in the long run, and they are readily available at low
price.

>3) Should I buffer the input with an op-amp?



Yes, get yourself a quad rail-to-rail op amp.
>   So maybe I should really use a (FET input?) op-amp between my
>   circuit's input connector and the analog input pin of the PIC.
>   That would also help me add some gain to the signal. What type
>   of (not too hard to find) op-amp would do you suggest me for
>   this use?

A rail-to-rail op amp, of which there are many to choose from will give you
all this. There are also the possibility of changing the gain as well,
another reason to have two way communication. Using a quad op-amp will allow
you to have up to 4 channels which I am sure you will find a use for, even
if you cannot think of one at this stage.

>4) Would it be wise to power everything off the PC's RS232? Together
>   TX, RTS and DTR may give me at least 20mA even on a portable PC,


No, get yourself a wall-wart, and properly filter and regulate it. You will
thank yourself in the long run - think in terms of being able to disconnect
the PC, and go do something else while your PIC keeps logging the data into
internal EEPROM, as a later feature you can fit. Then come back some time
later and download - another reason for two way comms.
>5) I will put the circuit board into an aluminium box. Should I
>   ground it then? Also the input and RS232 connectors? I'm afraid


You will need to have some sort on connection between the box and the ground
side of the PIC. It may not need a direct connection, you may be able to
have a 10k-100k resistor which should be a low enough impedance most of the
time, but you may want to make provision for a direct connection using a
switch to short out the resistor.

>6) Anything I should be aware of and possibly (probably!) I ain't?

See reasons above for two way comms, and possible enhancements like multiple
channels and un-attended data capture. All easily added with absolutely
minimal extra hardware, provided you think about it at the design stage.

>Of course I will share the whole project (schematics, PCB, and info)
>with the List once it's complete, running, tested and blown up! :D

I am sure James would appreciate another project he can list on the
piclist.com pages.

>Oh my God.. there's smoke on the bench.. gotta close this post now..
>hurry up.. bye!! ;D

Yeah, and here comes my boss ...



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