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A 28.8 modem is 28,800 BITS per second. Your 19,200 CHARS per second = 153,600 BITS per second. Quite a difference.
> -----Original Message-----
> From: Gerrit Jacobsen [mailto:jrt@xxxxxxxxxx]
> Sent: Saturday, May 22, 1999 2:19 PM
> To: Philippe; omega-list@xxxxxxxxxx
> Subject: More on serial ports
>
>
> Some good explanation on serial port FIFO issues I just came across -
> Note that this is an old posting - todays computer can cycle the IRQ
> more often.
>
> "Albert P. Belle Isle" <belleisl@xxxxxxxxxxxxxxxx>
> wrote about {Re: high speed modems ?????} in
> 'comp.dcom.modems'...
>
> ~> Most PC's can service between 1,000 and 1,200
> IRQ's per second. Multiply ~>this by the size of a
> 16550 buffer (16) and all you get is 19,200 cps.
> Isn't ~>this is a little short of the 23,040 cps
> advertised for the 28.8's? ~>
>
> ~Ed:
>
> ~If you set RxTrigger for the FIFO to only 1 byte,
> then there will only be ~1 byte for the CPU to empty
> from the UART when it goes to answer the IRQ.
>
> ~However, if you set the RxTrigger to a higher
> value, like the Windows ~comm.drv default of 8, then
> the UART will wait until the FIFO has that ~many
> bytes in it before raising the IRQ. That number of
> bytes will then ~be taken off in a burst when the
> IRQ is serviced.
>
> ~That's the design idea behind adding the FIFO
> buffer to the UART: to get ~around the very problem
> you mentioned.
> ==============================================
>
> I understand and totally agree. However, if you only
> have a total of 1,200 IRQ's per second available
> (BIOS limit) and you multiply this by the full
> 16-byte UART buffer size you come up with a 'maximum
> theoretical DTE rate' of 19,200 bytes/sec. This is
> about 3,840 bytes/sec short of the advertised 23,040
> rate for a 28.8-LAPM compliant modem. My RxFIFO is
> set at 14.
>
> Ed..
> _______________________________________________
>
> All my sheets are white except my colored ones!
>
> ** Common Sense Is Not Common **
>
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