Help with Commodore 128D User Port

From: Leif Bloomquist <>
Date: Mon, 11 Jan 2016 11:40:31 -0500
Message-ID: <>
Hi all,

I got my "Wifi Modem" device working with the SX-64, thanks for the
help!  (The trick was to lower my current limiting to 100 ohms).

I've found a new issue, unique to the 128D and related to the UP9600 hack.

The symptom is that with my device plugged in, the 128D's built-in
1571 immediately goes to an error state (activity LED flashing
constantly) and I can't even read the error channel (PRINT DS$ returns

This does *not* happen with a flat 128 and external 1571, so it's
something related to the 128D - I'm aware the 128D's 1571 has a newer
ROM revsion (3.1 vs. 3.0) and there is probably an electrical reason

Here's the schematic for my device:

I've found that if I disconnect Pin 5 ("Serial Port 1, from CIA #1")
it works. With the UP9600 baud hack, Pin 5 is looped back to Pin M

This is the root of the issue but I'm not sure why.  With other RS232
adaptors (like Jim's EZ232) that support the UP9600 hack, looping back
Pin 5 to Pin M (via jumper) does not cause the issue. Also, as
mentioned this doesn't occur in a flat 128 and external 1571.   So I'd
like to understand what the issue is before blindly adding a jumper.

More details on my modem:

Thanks for any help!


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 2016-01-11 17:00:49

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