The following is a collection of comments developed from hearsay,
random documents, experience, and technical specifications. So,
take it with a grain of salt. Order information from the various
manufactures if you want accurate information.
A very good reference book that I use is available from National
Semiconductor Corporation. It covers most of the SIO devices
(also called UARTs) that you will find in all PCs and clones.
The title of the book is Microcommunications Elements Databook.
The publication number (manual number) is 400066. I am not sure
that the book can be ordered directly from National
semiconductor. They may require that you get it from a
distributor. In any case, the address on the back of the manual
is:
National Semiconductor Corporation
2900 Semiconductor Drive
P.O. Box 58090
Santa Clara, CA 95052-8090
Tel:(408)721-5000
TWX:(910)339-9240
8250
As best that I know, the 8250 was the first SIO chip (integrated
circuit) that was used by the IBM PC and many clones. In my
opinion, it was a poor choice on the part of IBM. I feel many
superior devices, at comparable prices, were readily available.
At that time, I feel the 8251A or the 8530 would have been better
choices. I feel IBM used the 8250 to insure the PC would not
have synchronous communications capability in a standard
configuration.
From a hardware standpoint the 8250 is a relatively slow device.
It is advisable that programmers not perform successive inputs or
outputs to this device. It seems that software programs can load
the various registers of the 8250 faster than it can process the
information. The 8250 had a total of 7 registers. The
specifications state that 56kb is the maximum baud rate.
8250A
I believe the 8250A is the 8250 with some bug fixes. I have no
idea what the bugs may have been. A quick glance at the
specifications shows the speed of the 8250 and 8250A to be much
the same. The 8250A added an 8th register. This additional
register enables software to detect if an 8250 is installed. The
specifications state that 56kb is the maximum baud rate.
19
16450
The 16450 seems to be a speeded up version of the 8250A. There
is no direct way (that I know of) for software to detect the
difference between an 8250A and a 16450. I believe the 16450 was
developed to eliminate the need for software to insert delays
between successive accesses to the device. The specifications
indicate the 16450 is a much faster device than its predecessors.
The additional speed is only the speed at which the processor can
access the device. The maximum baud rate for the 16450 is still
stated at 56kb. However, I have been told by some people that
they have run the 16450 successfully at much higher speeds. I do
not believe there was ever a 16450A.
16C451
The 16C451 is a CMOS version of the 16450. CMOS is a term for
the material and manufacturing process used to make the part.
CMOS typically uses less power than other technologies. If you
are not designing hardware, you should view the 16C451 as a
16450.
16550 Non A
It is hard to find a 16550 (Non A). I was told by National
Semiconductor that they did everything they could to get all
16550s back. X00 (and XU) will detect a 16550 and tell you if
you have one. I am told that the 16550 was installed in early
PS/2 systems.
The 16550 was the first shot at a FIFOed version of the 8250
family from National semiconductor. However, I was told by
National Semiconductor that the FIFOs of the 16550 are not
reliable and they should not be enabled. X00 will treat a 16550
like a 16450. In this mode, they are reliable. National
Semiconductor would not provide me with a specification for the
16550. However, I suspect its maximum baud rate is the same as
the 16550A which is 256kb.
16550A, 16550AF and 16550AFN
In the manuals that I have, National Semiconductor does not
explain the differences between the 16550A and the 16550AF. I
suspect the AF part may have a few bug fixes. I believe the N in
AFN describes packaging, ceramic versus plastic, DIP versus
surface mount etc.
In the opinion of the author, there is no substitute for the
16550A (and its successors) in the 8250 type series. The 16550A
is compatible with software written for the entire family of 8250
20
type devices. Programs that are 16550A aware can provide much
improved performance over previous devices.
The maximum baud rate for the 16550A is specified at 256kb.
However, due to the hardware design of the PC et al, 115kb is the
maximum baud rate that can be programmed.
The 16550A can be plugged into the same socket that contains an
8250, 8250A or 16450. If your SIO expansion board has the SIO
chips in sockets, you can upgrade to the 16550A by simply
removing the old chips and replacing them with 16550As.
The key to the performance increase of the 16550A is its FIFOs.
It has 16 byte FIFOs for both transmit and receive data.
16550s Made by Western Digital
I have been told, but I have not verified for myself, that 16550s
made by Western Digital have a problem with their FIFOs when
working at 2400 baud or below.
16C551
The 16C551 is a CMOS version of the 16550AF. See the above
description of the 16C451 for a discussion of CMOS. Do not feel
you need to upgrade from a 16550A or AF to a 16C551. There is no
gain from an existing users point of view.
16C552
The 16C552 is two 16C551s on a single chip. These devices will
start showing up on multi-port serial I/O cards.
82510
I believe Intel is the only company that manufactures the 82510.
The 82510 is feature rich with several modes of operation. Its
default mode is to operate as a 16450. The 82510 has a 4 byte
FIFO for both transmit and receive data. A 4 byte FIFO is
sufficient to provide significant performance over a basic 16450.
The 82510 is small in size. Therefore, it is found in many lap
tops.
The 82510 is somewhat of a sleeper. I believe it would be much
more widely used if Intel had promoted it more. However, given a
choice between the 82510 and the 16550A, I would select the
16550A.
21
Appendix C, Multi-Port Serial I/0 Cards
I get many requests for information about multi-port serial I/O
card. I will describe the boards that I know about (or have
heard about) here. I will add to the list as I receive precise
information from users.
My personal experience is with two boards that allow multiple
serial I/O ports to share a single IRQ. The first board that I
experimented with was the Connect-Com 4 from Software Link in
Atlanta Georgia. The second board is the 4 PORT RS-232 CARD from
Decision-Computer International in Taipei, Taiwan.
Connect-Com 4
I found the manual for the Connect-Com 4 to be very confusing and
their tech support even less helpful. I was able to get the
board configured to use the standard COM1 and COM2 and the
defacto standard for COM3 and COM4. If you use the Connect-Com
4, I suggest that you use it in this manner. This means that
COM1 and COM3 will use IRQ4 and COM2 and COM4 will use IRQ3. The
Connect-Com 4 has an 8 position DIP switch for configuration.
The setting that I used was positions 2, 3, 7 and 8 ON (up), and
all other positions OFF (down). In retrospect the Connect-Com 4
seems a bit expensive. However, once configured properly, the
Connect-Com 4 worked without problem. Software Link can be
reached in Atlanta Georgia at 404-448-5465.
4 and 8 Port Boards from Decision-Computer
The second board that I have personally used, and the one that I
am currently using is from Decision-Computer. The manual is
written well enough that I did not need any technical support.
There are two boards of interest from Decision-Computer, one is a
4 port serial I/O card and the other is an 8 port serial I/O
card. I read somewhere in the documentation (which I can not
find at this time) that the boards are a clone of boards
available from AST.
The boards from Decision-Computer are configured by jumpers. I
use 4 ports of the 8 port card. I selected the (so called)
Compatible Mode. This means that the first two serial I/O ports
operate as standard COM1 and COM2 using IRQ4 and IRQ3
respectively. The jumper setting for Compatible Mode is clearly
shown in the manual (page 8 of my manual). The I/O port
addressed used in that mode are also clearly defined on the same
page.
A second jumper that must be set (JP2) defines the number of wait
states. I set the jumper in position 3 which means 2 wait
states. This was for a 20 mhz 386 system. This setting works
fine for me. More wait states may be required or less may work
22
on your system. The amount of time lost due to wait states for
serial I/O is very minimal.
The last jumper(s) for the Decision-Computer 8 port RS-232 board
is the selection of the IRQ(s) for ports other than the first 2.
I selected IRQ3 for the additional ports that I am using (I am
only using 4 of the 8). This means that the first port (COM1) is
using IRQ4, all of the other (3) ports are using IRQ3. Position
2 of JP3 (left side) is the jumper that I installed.
Now for the seemingly tough part. I ordered the board from
Decision-Computer in Taipei, Taiwan. Why? Simple, the price.
The 4 port version of the card is $36.00 US dollars. The 8 port
version of the card is $100.00 US dollars. I have not really
looked very hard, but I have not seen anything near this pricing
in the US.
With a few exceptions, I found ordering from Taiwan about as easy
as ordering from a US company. The exceptions are, they like to
use a FAX for ordering etc. and you will have to pay in advance.
I have ordered several items from Taiwan, paying in advance, and
I have not lost a nickel yet. Decision-Computer will send
brochures if you request them. They can be reached at the
following address (FAX is best):
Decision-Computer International Co., LTD.
4/F No. 31-3 Alley 4, Lane 906
Ming-Shen East Road Taipei, Taiwan
R. O. C.
Tel: (02) 766-5753 or 765-9782 or 796-5786
FAX: 886-2-7665702
TELEX: 16059 DECISION
To send a FAX from this area to Decision, I dial:
011-886-2-7665702
The best way to order is to have your bank wire the money
directly to Taiwan. Be sure that you specify Federal Express as
the shipper from Taiwan to the US. Federal Express will take
care of any Customs hassle. In most cases there will be no
customs fees. In the worst case that I have experiences, I had
to pay Federal 2 or 3 dollars in customs fees.
Stargate Plus 8 board from Software Link
Dan Fox originally helped me with the testing of multiple serial
I/O ports sharing a single IRQ. He informed me that the serial
I/O board that he uses is a Stargate Plus 8 Board. He purchased
the board from Software Link in Atlanta Georgia, 404-448-5465.
Dan has all 8 ports using IRQ3.
Omen Technology has received reports of problems with buggy 8250 type
UART integrated circuits in Leading Edge modem boards, serial port
interfaces, and computers. The defective chip logic affects high
performance software. Replacing the buggy chip with a newer chip
(NS16550FAN) corrects the problem.
The UM82450 chip in the Zoom 2400HC also appears to cause problems.
Zoom Telephonics advises that this problem is limited to their old
model. The old model is inentifiable by an external volume control.
You can identify an old chip by inspecting the serial interface board
for 8250 or UM82450 type devices. A service technician or hardware
hacker can help if you are not familiar with reading chip numbers.
Dsz users can identify old chips with the following software command:
EXAMPLE: dsz port 1 speed 2400 p
ZCOMM and Professional-YAM users can look at the printout produced by
the s command; old UART chips register as TYP 0.
The National Semiconductor NS16550FAN is a pin compatible plug in
replacement for 8250 and 16450 serial chips. Omen Technology PCDOS
software enables the FIFO buffer in the NS16550FAN to prevents loss of
data from poorly written device drivers, TSR programs, etc.
We are also using the Goldstar GM82C550 chip with a 9025 date code.
This Goldstar chip is plug compatible with the National chips. No
problems have been discovered with this chip.
The NS16550FAN has better electrical characteristics than the older UART
chips. 386 and fast 286 machines should use the NS16550FAN for best
results. Omen Technology software has been written to allow time for
the older UART chips to operate. However, there is a limit to how much
the software can rearrange chip accesses without resorting to
performance robbing wait loops. Some other software is not as forgiving
of slow chips.
Be sure to ask for the NS16550FAN. It is VITAL to get the National
Semiconductor "AFN" or "AN" chip. Other vendors' chips not mentioned
above won't do. Current Western Digital WD16C550 chips do not function
properly at low speeds.
As of this writing, the Goldstar GM82C550, National Semiconductor
NS16550AN and NS16550FAN are the only plug compatible chips that are
known to work.
The Hayes ESP board comes with two serial ports with NS16550FAN devices.
The ESP definitions for COM3 and COM4 agree with the industry standard
definitions used by Omen Technology. All option switches on the ESP
board should be OFF except those used to select the port number (COMn).
The following information comes from a number of people:
"I have located a small dealer in New York who has the cards with or
without the 16550. I believe the price is around $50.00. The company
is RJK Consulting and the number is 718-968-9167. The gentleman to speak
to is a Mr. RJ Kulman. ... I got this information off the "Interlink"
BBS system." Fm: Nick Braak [TRICON] 75156,1254
Fry's - 415-770-FRYS. This is a Silicon Valley electronic department
store.
Jameco Electronics at 415-592-8097 sells National NS16550FAN chips mail
order (credit card) in small quantities. Unfortunately, we have
received a number of reports that their serial i/o card is now supplied
with the infamous WD16C550 soldered in, even when ordered with specific
instructions to include 16550FAN chips.
Microprocessors Unlimited, 24000 South Peoria, Beggs, OK. (918) 267-
4961. The price for the chip is $13.50 plus shipping costs. No minimun
purchase is required.
Consolidated Computers 4 Shoppers Lane Suite 121 Turnersville, NJ 08012
(609)751-9595(voice) (609)751-3843(bbs) (609)751-3840(fax)
Arrow Electronics at 800-932-7769 (516-467-1000).
Willies Computer Software Company: 800/835-4832.
N.B.: Current Western Digital WD16C550 chips do not function properly at
low speeds. This has been confirmed with samples received directly from
Western Digital. WD is aware of this problem, but has not informed us
of a corrected chip.
14.9.1 Everex Internal Modems
Numerous reports have been received concerning difficulties with Everex
modem cards. Some of these cards contain buggy 8250 chips which are not
up to the demands of full duplex streaming protocols such as ZMODEM.
Replacing the buggy 8250 chip with NS16550AN chip corrects these
problems.
