|
PureBytes Links
Trading Reference Links
|
Gerrit Jacobsen wrote:
>
> 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..
The Netlingo IT dictionary at http://www.netlingo.com/ defines
the standard terminology for modem speeds as follows:
baud
In common usage the "baud rate" of a modem is how many bits it can send
or receive per second. Technically, "baud" is the number of times per
second that the carrier signal shifts value - so a 1200 bit-per-second
modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300
= 1200 bits per second). It is no longer fashionable, however, having
been replaced by the more direct "bits per second" or BPS.
Maybe this definition will help resolve the "speed" question. However,
the bottom line remains that standard com ports are basically not
satisfactory for any high speed communication needs.
Rod
|