View Full Version : Need to change the UART driving the COM port.

07-01-2002, 12:45 AM
SiSoftSandra advised me that the UART driving the COM port must have large (10-16+) internal data buffers. I didnt know I had any of those things. Can someone please explain in laymans terms.


07-01-2002, 09:16 AM
You shouldn't need to change your UART settings, as they are set to High and Maximum (2 settings) by default.

To see where they are (and change them if you want)

open Control Panel > Modems and click on the default modem, then click Properties.

Click the Connection tab, then Port Settings.

The UART settings should now open in a new box. REceive buffer should by default be set to High - if you have NO modem trouble you could set it to Maximum. The Transmit buffer by default should be maximum.

If you have modem connection problems, you can try lowering these settings to get improved connection, but I'm not too sure what exactly it does, as I've never used it, apart from changing the receive buffer from High to Maximum.

07-01-2002, 03:06 PM
I suspect that you might have 'older hardware'. If you have the DOS programme MSD, try it. In the COM window, at the very bottom of the information for each port it will say something like 8250 or 16540 or 16550. If you have a 16550 you have a buffered uart chip.

A UART is the serial to parallel converter used in serial ports.
The original PC used the NS 8250 chip which had only one buffer (and a bug meant it did not work properly). It required an interrupt of the CPU for each character received. The CPU was slow so the bit rate was slow. If the CPU did not ask for the character before the next one was received, characters were lost.

Later versions used the 16540 uart, which better results.

Later still came the 16550, (with a bug), then the 16550a (bug-fixed version). This has about 16 buffers, and interrupts only when 10 or so characters had been received, so the CPU could do other things without losing characters.

If you have one of the older ones you might have slower throughput with a modem attached to the port. If the chip is socketed you could plug in a 16550 chip if you can get one (DSE have been quitting them lately), but there may not even be a chip to replace. Mostly the chip is part of a 100 pin centipede package on a multi-I/O card.

If it works, you're best to leave it. You can get PCI cards which have 16550 chips, but an older computer is probably not worth upgrading.

07-01-2002, 04:04 PM
Thanx for that very informative response Graham. I almost understood what you were talking about.

In conjunction with your reply and the one from Mikes above, I knew where to look and what to look at. My machine reports it has UART of NS16550AN so presumably SiSoftSandra got it wrong when it analyzed my machine. Whew. (Sigh of relief)

Thanx both.