[RFC - Vladimir ?] USB/RS232 converter PL2303 small problem

[Aleš Nesrsta]

Hi,

I don't know who is developer/maintainer of USB serial drivers - I
expect it is Vladimir, so this e-mail is addressed mainly to him.

As I wrote in my older e-mail (Question: USB serial - device driver
debugging), there is some problem with PL2303. Not with driver but with
device itself (probably).

What is the problem:

If I put some stream of characters into PL2303, device stops accepting
data and answers NAK indefinitely after some time / some amounts of
character sent (256 exactly).

It happens only if PL2303 is connected to another computer. If only
null-modem cable is connected, it does not happen. (If no cable is
connected, it happens also but in this case it is caused by missing
handshaking signals which prevent sending -> output buffer overloading
happens, i.e. it is in fact correct situation.)

I discovered the reason - I forgot switch off "echo" on ttyS0 of PC
where was cable connected. I.e. every character sent by GRUB was
returned from connected PC back to input of PL2303 and input buffer was
overloaded.

But what is surprising for me - why stops PL2303 SENDING when INPUT
buffer is full ???
("Normal" serial port does not such thing in GRUB, it looks like it is
PL2303 device speciality. I did not test FTDI, I don't have it.)

Maybe such behavior can be prevented by some another pre-setting of
PL2303 device - but there is no detailed documentation about PL2303
device USB interface.

I thought also about some workaround - continuously check amount of
characters in input buffer in device but there is problem how to do it -
at least there is no related documentation of PL2303 as I mentioned
above.

Another possibility is periodical fetching characters from PL2303 but it
should be disabled when PL2303 is used as input for terminal etc. - it
looks too difficult to implement (and non-standard...).

I.e. there is probably no simple way how to prevent this problem but
such unexpected behavior of device should be known and probably
documented somewhere - where ?
Is there some place in source where to store HowTos, FAQs and some other
more or less important notes ? Or should I write this information into
driver source code ?

Regards
Ales