RE: faulty IEC serial port on Flat NTSC C-128

If it works in 64 mode, then my guess it is the fast serial shift register circuits. The MMU drives the FSDIR signal which sets up the direction of the shift register. You are looking at U8, U37, U16, U30, U58, U60, etc.
You could also maybe test this by removing pin 1 from the serial connection which effectively makes the drive slow serial like the way the 64 mode would see it.

Roberto

From: Terry Raymond [mailto:traymond160_at_gmail.com]
Sent: Monday, November 01, 2021 1:35 PM
To: cbm-hackers_at_musoftware.de
Subject: faulty IEC serial port on Flat NTSC C-128

Hi all,

I have a USA NTSC Flat C-128  STOCK:
This 128 powers up just fine, and it resets in all modes.
The problem is that all of my disk drives wont work with it,
it just pops up this error message:  DEVICE NOT FOUND ERROR?

I have run the C128 Diagnostic with the harness attached and it really doesnt mention the
IEC serial port as not working etc.

The CIA chip for the IEC circuit tests okay
I replaced the TTL Logic Transeiver Bus chip, socketed it.
I replaced all of the IEC circuit I believe switching Diodes?
I replaced the IEC Din serial socket.

With all of that I still get the error message.
All I can figure is maybe i pulled pads or traces on the TTL Logic Transiever chip.

  I just did purchase a good Oscilloscope and getting a crash course on how to look for
high/low stuck bits etc and how to trace down a bad IC.

Other than that has anybody else ever had this failure?

BTW in the Diagnostic though the User port tested bad) since the Diagnostic uses that cartridge in the user port.  So I cleaned the Edge connectors and then it passed.

BTW I do have the C-128 Dead test and Diagnostic.
and I do have 2 different types of the harnesses, I cant recall the person that made them, well one was made last year in Canada.

Terry Raymond