emdeded modem question

wanchope · Guest

Hello, friends.
Now i am developing a module to communicate with a black-box. The black box basically consists a PIC and a lease line modem-AM7911 of AMD. This black box is built by third party, and i could hardly do anything about it except connect its RIND and TIP signal out to my modem. My embeded modem is 73M2901CL from TDK.
The baud rate of the black box is selectable. They are 300bps (Bell 103) and 600bps (Bell202). And AM7911 acts as originate modem.
I set S30 register of my TDK modem to value 2, which supports Bell 202.
Then i send ATA from my pic to TDK, the connection is established with carrier 1200. Then i use my hardware rs232 to send data at baud rate 600 across to the black box. But the data i sniffed out from pin 26 of AM7911 is wrong. But if i send my data with baud rate 1200, then the data at AM7911 is correct.
I guess this may be due to the 1200 carrier. But what could i do at my side? Could i change the carrier rate? Is it determined by the originate modem?
Looking forward to you suggestions.
Wanchope

wanchope · Guest

Hi, friends. Let me re-phrase my problem.
Can the transmission speed (baud rate) be different from the carrier speed over the modem transmission? Currently my data in the receive end is corrupted.
many thanks

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

If your connection speed to your modem is slower than your career speed you may experience buffer over-runs (the modem will be receiving data faster than it can report them to you). You'll receive only some of the bytes and the packet will look corrupted.

wanchope · Guest

thx, Ali,
Then any way to resolve?

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

The best way is to speed up the link between the PIC and the modem. PIC's USART can handle up to 230kbps so this shouldn't be a problem.

Another solution is to bring down the speed that your modem connects to the other party's modem. This is not a good solution since it sacrifices speed. However if you want to go down this path, check your modem manual to see which AT command is used for limiting the modem speed. Different modem chipsets use different AT commands. From what I remember most Hayes compatible modems use ATFx and US Robotics modems use ATS27=x for this purpose.

wanchope · Guest

Thx, Ali,
But as i mentioned yesterday, i do not have access to the firmware of the other side. I only have the spec of it saying that its connection speed is 600bps for Bell202 standard. But i scanned through the AT command set of my TDK modem. There is only 1200bps for Bell202 and 300bps for Bell103. And there seems no other option for me to limit the carrier speed to 600bps.
What do you think?
Wanchope

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

Why don't you speed up the connection between PIC's hardware RS232 and the modem, to 1200bps?

wanchope · Guest

Actually i could do so. But the module i am developing is to interface with another module which is at the other side of the modem. Because it is developed by third party, and with a baud rate of 600, therefore i need to adapt to it if i need to communicate with it. Do you ever find any modem could support carrier 600? BTW, is the carrier determined by the originate modem?
This project is actually quite simple. just connect 2 devices through 2 lease line modems for communication. But the 'third party' i mentioned happens to be our competitor. Therefore i worry whether they provide us the 'real' spec.