On 12/06/14 20:17, Daniel Llewellyn wrote:
> On 12 June 2014 20:07, Steve Miller <stevem@???
> <mailto:stevem@seawasp.com>> wrote:
>
> I'd still avoid cheap clones of Prolific devices (frequently
> PL2303 without serial numbers). I have never had a problem with
> FTDI devices though they are a little more expensive and they too
> are being cloned now.
>
>
> Unrelated to the discussion on timers, I was wondering what the main
> difference was between the cheapo clones and those they're clones of
> and more expensive FTDI-based serial adapters? There seems to be quite
> a discrepancy between the two classes of devices, and I recently
> plumped with the FTDI-style because they seemed more like they were
> likely to be compatible but I have no idea why that may be so.
>
> Any insights, theories or myths would be interesting to hear not least
> just to put my mind at ease that I didn't spend over-the-odds for two
> cables (one with an RJ45 end to connect to my mikrotik router and one
> with a bog-standard RS232 connector)
>
> --
> Daniel Llewellyn
>
>
I am not an expert on USB and am happy to be corrected. In my
experience, one problem with the cheap Prolific clones is that they
rarely come with an EEPROM to store a serial number. A serial number is
not required for USB devices, but it helps Linux (and lesser operating
systems) distinguish between more than one of them. This might begin to
explain why Prolific clones are cheaper than FTDI devices.
USB serial/parallel devices are not actually proper 1980s physical
serial/parallel devices. A change in a pin status can take some time
to propagate. FTDI devices offer some kind of control over this, you
can vary it between 1ms and 255 ms at the cost of waking up your CPU
when nothing interesting is happening. I do not know if Prolific devices
can do this; I've never seen it implemented. The Linux FTDI driver was
adjusted some time ago to tell the chipset to use the minimum latency of
1ms rather than the default FTDI 16ms. Whilst this was for a good
reason, (MIDI I think) it was not the right way of doing it and it's
easy to patch out.
I have only anecdotal evidence but a small number of over-engineered
legacy non-Linux applications seem to access the serial port directly,
require precise timing and then get it wrong. On three occasions
replacing the unknown USB serial converter with one from FTDI fixed the
problem.
Cheap Prolific clones seem to work fine as long as your application
isn't too demanding and you've only got one of them. I've got four FTDI
serial devices attached to the server in the kitchen. They do not go wrong.
I have not tried any other USB serial devices, I know that Cypress do
one and Microchip have some on their PICs.
In my view Daniel certainly did not waste his money!
Steve
--
Please post to: Hampshire@???
Web Interface:
https://mailman.lug.org.uk/mailman/listinfo/hampshire
LUG URL:
http://www.hantslug.org.uk
--------------------------------------------------------------
