Interfacing PDA to USB

[Theo van der Merwe]

Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<zpZ7c.23072$h44.2967709@stones.force9.net>...

In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<LjA7c.22739$h44.2869201@stones.force9.net>...
In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
snip
Thanks for the feedback. I am used to a protocol like RS232 in which
you can (in principle) implement any protocol with software.
Unfortunately it seems that the USB interface standard uses a fixed
protocol (implemented by a chip set)?

Pretty much.
There is no reason it couldn't be done in software, it's just that the
protocol is so fast that it would require a dedicated CPU, especially
for higher speeds.

For some applications speed might be an issue, but I am perfectly
willing (while travelling of course) to wait longer for the transfer
of information (in other words for me it is not essential to
communicate at the full bandwith of the USB1.1 or USB2.0 spec). A

Nope.
You have to go at the bus speed (which may be only 1.5MBps) or nothing
will understand.

Maybe another option might be to investigate a RS232 to USB converter?
Another possibility could be an infrared to USB converter?

These are the wrong way round.
They add a serail port to a USB bus, NOT a USB bus to a serial port.
snip
Unfortunately, reality doesn't care about what we think is silly.


Is 'reality' perhaps related to Mr. Bill Gates? The very complicated
USB standard certainly seems to stiffle competition...

No it doesn't.
It's about as complex as it needs to be, to do all it needs to.
It's freely available, and downloadable by anything.

I think it is much too complex for my needs! For a PDA you don't need
to control 127 USB devices at a time. Also it is not necessary to work
at the full USB transfer rate (although it would be nice). In
principle I suspect one could aaccomplish most tasks with only a
single USB device at a time (which should make the USB standard much
simpler). Downloading the standard is not going to help much if one
needs a dedicated chip for USB communications! Designing a USB product
unfortunately seems to involve a big capital investment at present
(which is suited to big companies of course).

However, there are reasons why it's challenging, apart from the
fact that it's not done.

Many PDAs have little memory.
What would a 8Mb device do with a 24Mb scanned picture.

Is there perhaps a way to implement the USB protocol using a serial to
USB converter? As mentioned previously I am not primarily interested
in performance (although the faster the better of course).

No.

What would be the limitations preventing this from working?

Many PDAs simply cannot supply the current many USB devices will draw,
their batteries are too small (or the power converter which increases
in size weight and cost as power use goes up).
(most 2*AAA batteries simply cannot produce the 2.5W required by the standard
for more than moments)

Supplying external power should pose no problems? Can a memory stick
really draw 2.5W?

No, not in this case.

There are some USB standards for interfacing, which are generally
followed for mice, keyboards and storage devices. (search for HID and
mass-storage)

Do you perhaps have a specific reference for USB memory sticks and USB
hard disks?

As I've said, go look at http://www.usb.org/

I have already had a look, but couldn't find the standard on memory

sticks or hard disks.

Pretty much anything else requires a custom driver, and will often
make tradeoffs in the hardware assuming there is a fast CPU at the
other end.
These drivers are only typically written for one or two versions of windows,
and a few other operating systems if lucky.


Finding a USB driver for a PDA seems highly unlikely?

They are not written for them, no.

Thanks for the feedback.

However, there is a proposed (new?) standard for doing bidirectional stuff
for new hardware.
See http://www.usb.org/


Interesting - but is this different from making the device both a USB
host ('Master') and USB device ('slave'), perhaps with an option for
the user to choose which mode?
Yes, it's different.
You need to go and read the standards on http://www.usb.org/

Thanks for the feedback.

Is this perhaps going back to the concepts of the old RS232 (although
at a much faster potential transfer rate for USB)?

No.

It would indeed be handy, I'd like to be able to plug my camera into
my MP3 player, and save pictures.

If your MP3 player is organized as a 'memory stick' I suppose that
should be possible in principle (the camera could be the USB host or
'Master' and the 'memory stick' the USB device or 'Slave, or vice
versa).

 

[Ian Stirling]

In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:

Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<zpZ7c.23072$h44.2967709@stones.force9.net>...
In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<LjA7c.22739$h44.2869201@stones.force9.net>...
In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
snip
Thanks for the feedback. I am used to a protocol like RS232 in which
you can (in principle) implement any protocol with software.
Unfortunately it seems that the USB interface standard uses a fixed
protocol (implemented by a chip set)?
snip
Is there perhaps a way to implement the USB protocol using a serial to
USB converter? As mentioned previously I am not primarily interested
in performance (although the faster the better of course).

No.

What would be the limitations preventing this from working?

The same sort of limitations that mean you can't generate electricity
by putting hot water into an electric kettle.
It's the wrong way round.

There are some USB standards for interfacing, which are generally
followed for mice, keyboards and storage devices. (search for HID and
mass-storage)

Do you perhaps have a specific reference for USB memory sticks and USB
hard disks?

As I've said, go look at http://www.usb.org/

I have already had a look, but couldn't find the standard on memory
sticks or hard disks.

http://www.usb.org/developers/devclass_docs/usbmass-ufi10.pdf
http://www.usb.org/developers/devclass_docs/usb_msc_cbi_1.1.pdf


Memory and hard disks are generally talked to in the same way.

 

[Theo van der Merwe]

Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<yDg8c.26070$Y%6.2949341@wards.force9.net>...

In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<zpZ7c.23072$h44.2967709@stones.force9.net>...
In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<LjA7c.22739$h44.2869201@stones.force9.net>...
In sci.electronics.design Theo van der Merwe <ntvdml@iafrica.com> wrote:
snip
Thanks for the feedback. I am used to a protocol like RS232 in which
you can (in principle) implement any protocol with software.
Unfortunately it seems that the USB interface standard uses a fixed
protocol (implemented by a chip set)?
snip
Is there perhaps a way to implement the USB protocol using a serial to
USB converter? As mentioned previously I am not primarily interested
in performance (although the faster the better of course).

No.

What would be the limitations preventing this from working?

The same sort of limitations that mean you can't generate electricity
by putting hot water into an electric kettle.
It's the wrong way round.

There are some USB standards for interfacing, which are generally
followed for mice, keyboards and storage devices. (search for HID and
mass-storage)

Do you perhaps have a specific reference for USB memory sticks and USB
hard disks?

As I've said, go look at http://www.usb.org/

I have already had a look, but couldn't find the standard on memory
sticks or hard disks.

http://www.usb.org/developers/devclass_docs/usbmass-ufi10.pdf
http://www.usb.org/developers/devclass_docs/usb_msc_cbi_1.1.pdf


Memory and hard disks are generally talked to in the same way.

Thanks for the information. I have had a quick look at the
specifications, but cannot quite tell how to send information to the
hard disk (or memory) from the specifications (which seems to be
written for a USB developer, not end user). Assuming the simplest case
of a single USB device ('Slave') connected to a single USB host
('Master'), what would be the (minimum) USB commands necessary to read
a single byte at address X from hard disk (or memory)?