Help with SX64 User Port?

From: Leif Bloomquist <>
Date: Mon, 12 Oct 2015 21:09:16 -0400
Message-ID: <>
Hi all,

Some of you know I've been tinkering with an Arduino-based Wifi
"Modem" for the Commodore 64 etc.  (Pics here:

It's almost ready to go, however, one issue is that it doesn't work on
the SX64, and it's the same issue on at least two SX's.

The symptom is that while the Arduino receives serial data from the SX
fine, nothing sent by the Arduino is received by the SX.   I've also
tried going directly to the Wifi board (which is also serial), same
issue.   It works fine on a normal 64/64C though.

I'm aware that there are some differences with the SX User Port
compared to a normal 64/64C  (one of the 9VAC lines is "grounded",
from what I've read).  I don't have a lot of electronics background so
I could really use some help.

An EZ232 works just fine on my SX, so it's clearly possible.  This is
all at 2400 baud.

Some theories I have:

-I tied all the grounds (both signal and protective ground) on the
user port together on my board, could there be an issue from the 9VAC
coming through the ground?   (I'm not using the 9VAC pins themselves,
they aren't even connected.)

-I'm going through 330 ohm resistors to protect the CIA  (i.e. in case
User port and Arduino pins are both outputs).  Could this be putting
the TTL signal out of spec on the SX, but not on the 64(C)?

I'll post a schematic momentarily, but wanted to ask if anyone has
some advice or info as to what the problem might be?


Leif Bloomquist | | +1 416-737-2328 | Check out
my blog!

"Every choice, no matter how small, begins a new story." - xkcd

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Received on 2015-10-13 02:00:08

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