Troubleshooting some data transfer over the User Port

I am in the process of troubleshooting the alteration of some data in my USBhost-64 replacement. I thought to share the details of the data alteration in the hope that it might ring some bells with others while I give it a thought.

Here’s a few couples of bytes that result in the second byte being read by the C64 as 0xFF:

01 FA
20 BF
38 EF
02 F7
10 FD
20 FD
04 FC
40 DF
00 CE
11 FE
05 FD
05 EF
09 FE
03 DF

There isn’t a pattern about their position in the data stream, just the fact the first byte seems to be <0x60 and the second one >0x60, although such couples are NOT always altered. In other words: if a couple is altered, then it satisfies the above condition, but if the above condition is satisfied the alteration might not happen. E.g.:

0B DF <- not altered

My initial suspicion was around bus signals, e.g. reading the data out of the bus when the bus is still set as input or similar. However, if I put some code in place to delay reading data and possibly cope with such a scenario, it does not make any difference. Besides, the second value in each couple is NOT altered elsewhere, e.g.:

7D DF <- not altered

Any thoughts before I dig deeper into this? The first one who solves the mystery gets a free device!

About Luigi Di Fraia

I am a Senior DevOps Engineer so I get to work with the latest technologies and open-source software. However, in my private time I enjoy retro-computing.
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