Re: MODEM driver

jituelect wrote:
Hi all,

I am Working on a CDC ACM interface and using WDF.

While installing Modem Driver TAPI sends ATQ0E0V1 . My continous reader on
bulk read is getting response 0xd 0xa 0x41 0x54 0xd 0xa (OK) i.e 6 bytes.

Now TAPI reads the data single byte at a time. but instead of 6 it is
sending seven 7 request and my driver is hanging as7th read request is
pending .

Real and emulated serial-port-like drivers are supposed to just return
the available bytes (but at least 1) in this situation, then wait for
the caller to call back again for more. This is true both on most
Windows and non-Windows systems.

Thus when a port is in active use, there is usually always a pending
read waiting for the buffer to grow beyond 0 bytes and then returning
with whatever it has as soon as something arrives. This way the reading
thread is blocked in the driver and the system can wait at 0% CPU for
the next character to arrive, maybe in a millisecond, maybe in a year.

The classic example is a service modem which is listening for an
incoming call from a service technician. TAPI (or some other process)
will be waiting for a read of the first byte of the "RING" message, and
will be blocked in that wait until someone calls the modem, which only
happens in a rare emergency.


--
Jakob Bøhm, M.Sc.Eng. * jb@xxxxxxxxxx * direct tel:+45-45-90-25-33
Netop Solutions A/S * Bregnerodvej 127 * DK-3460 Birkerod * DENMARK
http://www.netop.com * tel:+45-45-90-25-25 * fax:+45-45-90-25-26
Information in this mail is hasty, not binding and may not be right.
Information in this posting may not be the official position of Netop
Solutions A/S, only the personal opinions of the author.

.

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