USB Inquiry

[MicroManiac]

Hello,
Does anyone know how many USB devices can be attached to the same computer at the same time using the microchip driver?
_________________
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."
Albert Einstein

[Ttelmah]

Er.
The microchip driver, couldn't care a jot.
126 devices is the normal maximum in USB, on a bus.
Performance will degrade significantly well before this.
It is the master, and the timings of the bus as a whole that sets the limit, not slave drivers.

Best Wishes

[MicroManiac]

Ok this means that, theoretically, I can put 127 devices on the same PC.
But in order to do that, each should have its own PID?

Is that correct or it is not necessary to change the PID?
_________________
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."
Albert Einstein

[FvM]

No, you can have many instances of a devic type respectively a VID&PID combinations.

There are basically two enumeration mechanism available. Typically, an USB device ha no serial number. It will than remembered by the device manager on a connected port base. Or the USB device can have an unique serial number, then it's identified by this number.

[MicroManiac]

Thank you for your answers.
I am using the USB in BULK mode, and the identification of the multiple devices should be done in software.
Do u have any idea how this can be done?
_________________
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."
Albert Einstein

[Ttelmah]

Note also, that I said _126_, _not_ _128_.

Best Wishes

[Ttelmah]

Some more comments.
The root hub at the PC, normally brings you down to 126 downstream devices.
How you scan/identify is completely down to you. For example, I have a system, which uses a number of units on the USB bus. A simple system has one, but more complex units may have several others present. One is always the 'core' unit, and it has a difference in it's connections, that is detectable by reading a value from the unit. The others have connections between them, that again allow them to be identified. When the PC software starts, it reads the registry, and identifies the names being used on the USB bus, for all devices with the specified VID/PID. It then reads them 'in turn', and uses the diferences in the values read to identify which is the core controller, and what order the other devices are connected in. Because the detection is dependant on the connections, you can swap a board at any time, reset the software, and it automatically redetects the new layout.
Remember USB, is a _local_ bus. It is designed to go only 5m on a segment, and though you can have hubs as repeaters, these again use up addresses. It is also not isolated (this can be added). For anything reasonably industrial, wanting to address a lot of devices, you are much better off going to something better designed for longer distances. The USB system mentioned above, is only the local connection blocks. The units themselves talk to other slaves further away, with these using an RS485 bus. Also other systems can be cross connected into different rooms, and data archived. These links use Ethernet, rather thean USB. Don't design with USB, and then find yourself regretting using a bus that is not designed to really do what you need...

Best Wishes

[MicroManiac]

OK,
Thank you all for your help, u were very useful.
I think now i have a very good idea on what should i do
_________________
"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."
Albert Einstein