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USB - Serial Speed

 
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Deiphos



Joined: 12 May 2004
Posts: 4

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USB - Serial Speed
PostPosted: Wed Jun 20, 2007 3:57 am     Reply with quote

It's just a simple question I can't solve on myself...when using the usb to serial, anybody knows wich speed data is being sent?
Any speed I select on the hyperterminal catches any data sent, so How can I know the speed?

Thanks to all.
Ttelmah
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PostPosted: Wed Jun 20, 2007 4:21 am     Reply with quote

It is physically 'sent' at 12Mbps (assuming this is on a normal USB 'fast' interface).
However a single device, is only itself connected to the USB and receiving data for a small fraction of the time. The actual fraction available, depends on the driver being used, and the other devices on the bus. This normally limits the maximum amount of data that can actually be moved in a second, typically to around 1Mbps (though things like HID drivers, limit much earlier at under 1/10th this).
When you set the 'baud rate', on a USB COMM driver, it really does nothing to the local rates on the USB bus. Instead it sends a command to the slave device, telling _it_ what rate to use. It can pay attention to this, and adjust it's behaviour to give the required rate, or ignore it completely. Normally it'd adjust the speed of it's external interface to the required rate, but it doesn't have to.
You can see the 'set' rate, from the program you use, or from device manager in control panel.
The physical maximum usable, will depend on the abilities of the chip involved, and the driver in use.

Best Wishes
Deiphos



Joined: 12 May 2004
Posts: 4

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PostPosted: Wed Jun 20, 2007 5:13 am     Reply with quote

If I connect an RS232 device and I set a wrong baud rate data won't be displayed, but with USB to UART they do, at any baud rate, thats why I think that data speed is not dependant on what I select and they are being sent at some pre-determined speed that I would like to know.
I know that maximum speed for USB 2.0 is 12 Mbps, but the maximum speed for USB to UART is the same?
Ttelmah
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PostPosted: Wed Jun 20, 2007 7:10 am     Reply with quote

There is no relationship between the speeds.
The data to be sent to the USB-serial converter, is assembled as a 'packet' by the driver at the Windows end, and sent over the USB link, at the speed that the interface is running at. This will depend on the negotiations between the slave chip, and the master, but will normally be 12Mbps. The slave device, then has the 'job' of putting the data in this packet, 'out' over an RS232 link (or receiving the data over such a link, and assembling the packet to go back to the master). The speed it uses for this, can be indeterminate. The USB driver sends configuration packets 'saying' what rate should be used, but the device is under no obligation to use these rates.
You cannot tell from the PC 'end', what rate is physically being used. You can only tell what the slave has been _asked_ to use.

Best Wishes
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