Re: serial link high level protocol thoughts?

From: Jim Brain (
Date: 2008-11-07 19:05:10

Marko Mäkelä wrote:
> In the summer of 2003, around the time I became a father, I implemented
> serial bus interface for the C2N232 adapter.  By December, I had it mostly
> working, except that it couldn't talk to a 1541.  It would emulate up to
> 32 devices.  I tested by issuing LOAD, SAVE, OPEN, GET#, PRINT# commands
> on the C64 or VIC-20 and by sending appropriate control/data sequences
> to the modified C2N232 over RS-232.
Yep, I remember looking at your protocol and discussing this back in 2005.
> I made the interface very low-level.  The firmware running in the C2N232
> takes care of asserting the bus or responding to ATN.  It can act as a
> controller or as up to 32 devices.  (You tell it which serial bus device
> numbers it will emulate.)  It will relay all commands and data over the
> RS-232 link.  The software running on the PC would act as appropriate
> (emulating disk drives and printers, or emulating a host that talks to
> real serial bus hardware).
I implemented the low level protocols in 2005, and found that RS232 
communication interfered with transfers resulting in corrupt data.  I 
was not able to resolve the issue.  I remember one example:

64 would open a file for reading.
low level protocol would send 2 command bytes + filename.
PC would open file and send ack back
64 would open channel for reading
low level protocol would send 2 command bytes
PC would respond with 254 of data
64 would issue GET#,A$
low level protocol would send 1 byte of data back
64 would immediately bring ATN low
low level protocol would then send ack to PC telling it that 1 byte had 
been sent
64 would send 2 more command bytes...

Under load, the link would get saturated with chatter and the system 
would often corrupt data.  Another example generated the same issue and 
corrupted file loads.  While I am no IEC expert, I finally gave up. 

To contrast, I implemented a high level protocol the other day that let 
the uC maintain buffer positions and only sends/receives data in blocks, 
and it works fine.

I also support fastloaders, which further complicates a low level 
protocol, but has no impact on the high level protocol.
> Wolfgang Moser just made me aware of the sd2iec a couple of days ago.  It
> feels like a good idea.  I was wondering if you could implement some tape
> emulation in it.  However, that would require some user interface (LEDs or
> display and some buttons) for selecting the image to read or write (virtual
> tape control; play/rec/fast-forward/rewind).  Maybe not worth the trouble
> and additional hardware cost.
uIEC uses the sd2iec firmware, and it's the same firmware I hacked to 
implement my initial high level protocol.  It works fine, even at 

Thus, while I agree there might be some utility in a low level protocol, 
I'd rather concentrate my efforts on a high level protocol at this time.


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