Dan Rhodes said:
Hi Folks -
Since USB is a differential bus, is it possible to get the same range
performance as RS-422 if the devices do not draw power from the bus?
I have an application where I need to connect USB devices 500+ feet
apart, and would rather avoid the expense of something like a
USB-RS232/422 converter IC based device if possible.
It seems like since the Data+ and Data- lines are differential, the
same reasons why RS422 works should apply.
Several parts to this.
The signals are differential, but also bi-directional. There are very tight
'constraints' on the overall time taken for bus 'turn round' and signal
delays.
It is possible, by adding some 'glue logic', to implement a transciever,
that uses a 'better' signalling standard, and extends the range to perhaps
200 feet, but beyond this the timings fall apart (a couple of commercial
products exist to do this). Hence you can't go the sort of distance you need
with a passive transceiver device. The commercial units that go further,
instead behave as a single device 'hub', and talk unit-unit, then regenerate
the USB bus at the far end. Remember that each USB device, itself needs to
have an ID and drivers, so to build a DIY unit, will involve either using a
standard chipset, and assuming that drivers will be available, or either
using a 'trick' bodge to an existing hub chipset, or building your own
devices, writing the drivers, and registering this as a USB device...
Even on the commercial devices, many of the cheaper units, still have
reliability problems.
Look at units like the Icron ExtremeUSB devices. Some are available with bus
lengths up to 500m, using multi-mode fibre optic cable ( 'EverGreen', alse
do versions of these as their 'FireLINE' products - your length is longer
than I have found any of the 'Cat5' based solutions to work reliably...). No
solution is going to be cheap.
As a completely 'lateral' solution, you could use a 'single board'
industrial PC, at the remote end, to scan the USB device, and then send the
data (depending on rate), over another medium (unfortunately, the length is
such, that even ethernet, would require repeaters).
A 'USB-RS232/422' converter device, would not normally do what you want.
Most are only available as 'slave' devices, and the data rate required by
USB, will bring the bus length supported down below the distance you
require.
Best Wishes
