Weird sprious input

[Johnny Billquist <bqt%softjar.se@localhost>]

I've rewritten the console driver for the VAX some, in my effort to 
support all the console ports of a VAX-86x0.
One of those ports is the EMM (Environment Monitoring Module), which 
both report problems, and can respond to queries.

After adding this one, I seem to have hit upon some weird error in the 
tty driver. So I thought I'd ask here if anyone knows what situation or 
circumstances might cause this, if anyone have seen this before, and any 
suggestions on what to do. I really do not feel like starting to hunt 
and debug the whole terminal subsystem.

The scenario is like this:

I've added interrupt counters to rx and tx for the VAX. I've extended 
the console driver to access several different ports, selected by the 
minor number of the device. In the machine, all ports work through the 
same registers and interrupt vectors. The receive register is 32 bits. 8 
bits is the data byte, and another 4 bits is the actual source of the data.

I've written a small user-mode program that opens the device that map to 
the EMM, and I send one byte out on that port. That byte is a request 
for status of the EMM. The EMM should respond to this with a total of 34 
bytes.

The interrupt counter matches this. I do get 34 interrupts after sending 
the byte out.

However, in the user program, I sometimes get 34 bytes, but sometimes I 
get more than 34 bytes. The exact number is somewhat random, but range 
from about 44 to 56 bytes. The additional bytes do not have any 
corresponding interrupts which caused them, so I have no idea where they 
come from. Looking at the data I get back, I can identify which bytes 
actually comes from the EMM, and which bytes have been inserted into the 
stream by some weird entity. The place in this sequence of 34 bytes 
where the spurious bytes are inserted is random, but seems to happen in 
the latter half of the expected 34 bytes.

The values of these random bytes sometimes looks like they follow some 
pattern, but I have had totally strange values returned as well. Adding 
an aprint_normal inside the interrupt handler, to just show what values 
are received, and from where, seem to affect the system enough that I 
don't get the random bytes. That would suggest some timing issue.

The actual data expected is binary, and not human readable text. The 
randomly added data is also binary junk, and not something readable.

An example is this:
Good: 01 20 b5 00 b6 01 b6 02 49 83 49 84 bd 85 bc
Bad: 7f e4 eb ff 7f 49 54 7d 7f 58 03 01 00 b6 00 00 00 2e 04
Good: 86 00 07 9e 08 af 89 c7 0a db 8b 1f 8c 2c 8d 26 8e 27 8f

(All received exactly like this, the bad bytes in the middle.)

As the EMM is another small computer, the response back over the serial 
line might be high. I don't know at what speeds the actual communication 
takes place. But it's not like a human sitting at a normal asynch port 
and typing away.

Anyone seen anything like this before? Or have any good ideas on how to 
debug this?

        Johnny