USB issues / signal quality?

Soldato
Joined
18 Oct 2002
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6,672
We use a lot of USB devices at work on our equipment, namely USB-4 way RS232 adapters and a special Devasys board (which I don't know much about as it's not my area - it's an I^2C board).

Anyway, we are having more problems than we would like where the PC just doesn't talk to this Devasys board properly. The connection is this:

PC -> USB lead (often LONG, like 3m) -> USB B type female connector on device -> 2-way unpowered USB hub inside device -> 30cm USB lead to Devasys board.

Now this is a pretty convoluted connection setup - could it impact the signal quality and cause the device to not work properly? It appears in Device Manager OK and drivers are OK, but I cannot get it to communicate properly at all.

In the past, we've had garbled data coming back (jumbled alphanumeric characters for example, when we would expect clear strings like words or numbers), and sometimes we can't connect to it at all.

Sometimes swapping the main long USB cable to another one helps, sometimes not.

Anyone experienced with USB devices in this way?
 
IIRC the maximum length of a USB cable (according to the spec) is 5m. I have a 4.5m cable around somewhere I used to use for my printer, I don't recall having any problems with it.

That said maybe your USB controler and/or the hub doesn't like the cable length even though the specs say it should work. Try using a shorter cable or a different hub (plug your current one into the new one using a short cable if nessecary).

edit: just noticed the bit about garbled data... you'd think USB would have safeguards against that (e.g. CRC checks), maybe it's getting garbled on the way to the serial adapter from the device?
 
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The theoretical limit to USB is infinity, but the signals have to make it from the device to the PC in 5ms, I think.

From what you've said it might be just too much to communicate through.

Im a little unclear on the hardware you're using what device has the 2 unpowered ports in it? Are you converting USB to RS232?
 
Im a little unclear on the hardware you're using what device has the 2 unpowered ports in it? Are you converting USB to RS232?

It's an in-house custom-made piece of hardware. It has a two port unpowered hub internally so that it only needs one USB connection externally. Inside, this two hub provides a link to the Devasys IO board and a 4-way RS232 converter.
 
Get a Powered hub and try that out. If The USB lead is only pluged in to the motherboard then its only pushing 5v (yes thats normal) But its pushing it a very long way.
 
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