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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.
You are right I overlooked this.
I mainly posted this old thread from dejanews because it described
the mechanism between IRQ, computer cycles and theoretical
throughput.
Sorry for this.
Gerrit
> 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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