PCMCIA modem fails when sending AT commands without delays

From: samraj (sam_ak_at_rediffmail.com)
Date: 11/17/04 

Date: 16 Nov 2004 23:35:51 -0800

We have tried to debug the Sierra wireless modem AirCard 750(PCMCIA)
interfaced to our PowerPC 8270 based target through TI PCI1520 (PCI to
PCMCIA controller).
We used the "Linux Configuration for AirCardŽ 55x and 7xx" Application
note 2170005 dated June 3 2002

(http://mycusthelp.com/sierrawireless/supportkbitem.asp?sSessionID=&Inc=192&sFilA=FAQ%20Category&sFilB=Products&sFilC=&KEY=linux)

for testing. We have configured the PCI1520 controller to access the
registers on the Seirra wireless pc card. The Sierra wireless card
emulates a 16550 UART serial port interface.

The wireless modem is configured with the following parameters

1.Baud rate : 9600
2. Parity : NO PARITY
3. Data transfer: 8bits
4. FIFO enabled
5. We used a simple program without any OS.

We succeeded in dialing an external phone by sending ATD command and
also we were able to send SMS message using extended AT commands with
the following limitations.

1. The above operations were successful when the AT commands were sent
with a delay (approximately 1 sec) between each character written to
the virtual serial port transmit register.
    The modem responds with "OK" or relevant success message.

2. These operations fail when the delays are removed. The modem
responds with error messages like "NO CARRIER" and "+CMS ERROR: 513".

We thought that the errors could be due to baud rate mismatch. Hence
we have tried with baudrates : 19200 and 38400 but the test did not
succeed.

Why don't the AT commands pass without inserting delays after each
character sent?.

Thanks in advance,
Sam

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