Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!uunet!basfpac!stratsft!dwayne
From: dwayne@stratsft.UUCP (Dwayne Bailey)
Newsgroups: comp.dcom.modems
Subject: Re: UARTS and Buffers...
Message-ID: <131@stratsft.UUCP>
Date: 3 Apr 91 01:12:16 GMT
References: <9104010808.AA05643@ucbvax.Berkeley.EDU>
Organization: Strategic Software, Allen Park, MI
Lines: 29

In article <9104010808.AA05643@ucbvax.Berkeley.EDU> ST7021@SIUCVMB.BITNET writes:
>Recently someone brushed on two different UARTs, the 16450 and the 16550.
>Can someone give me an idea of what the difference between these two chips
>is? It seemed that what brand you get makes a big difference.

The 16540 and 16550 (as well as the 16550A) are all UART chips from National
Semiconductor.  They are designed to replace the 8250 used the the original
PC to gain a higher throughput that 9600 bps on AT class machines.

The primary difference between the chips lies in the use of a FIFO buffer.
The 16540 is unbuffered.  The 16550 and 16550A have two 16 byte buffers
(one for transmission, one for reception.)  This greatly reduces the overhead
to the CPU, as it doesn't get interrupted for every character.  Unfortunately
the 16550 has a bug in the receiver FIFO, and can sometimes give extra
characters that were never sent!  The 16550A fixes this problem.  Thus the
16550 should not be used if FIFO mode (that is from the NSC data book on the
16550A.)

Note that some software which detects the 16550 FIFO does not detect the
difference between the 2 chips (QModem 4.0 under DOS, for one.)  Thus, your
communication may contain sporatic errors.  If you have a 16550, replace it
with the 16550A if at all possible.

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..!uunet!basfpac!stratsft!dwayne

Dwayne Bailey      +  Strategic Software is ME, so my opinions ARE those
Strategic Software +  of my company!
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