PCMCIA modem fails when sending AT commands without delays

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 AirCard® 55x and 7xx" Application note 2170005 dated June 3 2002

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. Parity : NO PARITY
2. Data transfer: 8bits
3. FIFO enabled
4. 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.

1. 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