Info on the 16550a UART There has been much talk on the echo conference "HST" about a new UART chip. A UART chip is the 40 pin chip on your serial prort. UART means Universal Asynchronous Receiver Transmitter. This chip converts an 8 bit byte from parallel to serial and from serial to parallel. Your computer sends the byte on a PARALLEL 8 bit bus to the serial card. The serial card has to send it SERIALLY to the modem. And at the same time it adds a START BIT and a STOP BIT. If you are using a 7 bit format, the UART chip can also add the parity bit, which can be even, odd, zero, forced to 0, or forced to 1. It can also add 2 stop bits instead of 1 stop bit. And in the case of certain baud rates, it can add 1.5 stop bits instead of 1 stop bit. Well, a problem cropped up with the HST modem. To gain the maximum efficiency of this modem, you have to set your serial port to 19,200 baud. Yes, that is 16 times faster than your normal 1200 baud modem. Now most computers, even XT's, can keep up with that. But there is a problem caused by some EXPANSION boards like Perstor disk controllers, and Network Interface cards (like Ethernet) that hog, or tie up, the interrupts, or IRQ's. If the other board hogs the interrupts longer than one character time (1/1920th of a second) then the character that is coming in over the serial port is LOST and gone forever. With a 2400 baud modem, you have a 1/240th of a second 'slack' between characters. (Remember, 8 bits, plus one start bit, plus one stop bit, give you 10 bits (not 8) when talking about modems and serial ports. So you divide bits per second (bps) by 10 to give CHARACTERS per second (cps). So these problems didn't crop up with modems up to 2400 baud. But when running 9600 baud and 19200 baud, they did show up. What was needed was a way to "hold" those characters until the computer could catch up with the serial port. One way of doing that was making an expensive INTELLIGENT serial card. You could put some local memory on there and a small microprocessor like a 6052 or 8088. But National Semiconducter went even better! Instead of making the whole card smarter and a whole lot more expensive, they put all the smarts on the UART CHIP! The new UART chip, called the 16550, has a 16 byte FIFO buffer. FIFO stands for "first-in-first-out". With this buffer, the computer can be busy doing something else for up to 16 character times, and no characters will be lost. The original UART used in XT computers was the 8250. This was kind of slow, but adequate for the 4.77 Mhz bus speed of the XT. With the 8 Mhz bus speed of the AT, a faster chip was needed so they used the 16450. These 2 chips are functionally identical, and are pin compatible. They have 40 pins each. Actually, you will see many AT computers with a 8250 UART on their serial port. The 16550 will work on any serial port that now has an 8250 or 16450. You just replace it. But you also need some special software to activate the buffer. Otherwise, the 16550 will act like a 16450 without the buffer. There is a program called 16550.EXE which tells the UART to turn on the FIFO buffer, and then the UART is just transparant to whatever communication program you are using. I have been told, on a TELIX support conference, that Telix 3.11 has 16550 support built in. I don't know if other popular telecom programs such as Procomm+, Qmodem, and other have it or not. There was a user on a local BBS who was having problems even downloading at 2400 baud because he was using Desqview, and running several programs simultaneously. He installed the 16550, and his problems went away. Another local sysop using an HST didn't even have any of the "problem boards" mentioned later in this article. He installed the 16550, and then was able to consistently use 19.2K baud with no problems. His transfer efficiency rose from 96% to 115%. You can obtain the new 16550a complete with new Multi COM card. ($65.00 plus s/h) from Node 66 East BBS 914-426-0729. Here are some situations where you *might* benefit from a 16550 UART while doing high speed transfers with the serial port: 1) You have a Perstor controller with an older ROM on it. I have read that some rom versions take care of the IRQ hog problem. 2) Your computer is on any kind of network. Network interface cards are notorious for hogging interrupts. (Tho' not all do.) 3) Your computer is an AT and you are using EXTENDED memory. The AT has to do a reset and takes a lot of time to switch from protected mode to real mode when its done with accessing extended memory. (Tho' not all AT's have this problem.) 4) You are running a multi-tasking software like Desqview or DoubleDos and are trying to do communications in the background, and lose characters when switching windows, or have too many background tasks/windows active at the same time. 5) You are a sysop with an HST modem and are getting less than 100% efficiency on your mail transfers. Many factors affect this, so a 16550 *may* or *may-not* increase your efficiency to over 100%.