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Technical Note : FAST SERIAL PORTS & 16550 UARTS Author : Michael Spalter Email : segcom@cix.compulink.co.uk Origin : United Kingdom, EC Last Modified : 6th June 1994 Status : (c) 1994 Michael Spalter * PLEASE UPLOAD THIS FILE TO BULLETIN BOARD AND OTHER HOSTS * The filename should be FAST550.DOC or FAST550.ZIP etc. NOTE: This document may be distributed freely providing that all credits remain intact and that it is not modified in any way. This document is provided for information only and no support or warranty is offered by the author or publisher. NOTE : SEE THE END OF THIS DOCUMENT FOR 'UART' VENDOR DETAILS --------------- C O N T E N T S --------------- 0) INTRODUCTION 1) What is a UART ? 2) Why the sudden craze for high speed cards ? 3) Why aren't normal PC ports good enough ? a) Internal PC Card Modems 4) How would I know if I need a faster port ? 5) Okay, I want one ! What should I buy ? 6) Is there just one type of 16550 UART ? 7) Replacing a soldered 16450/8250 UART 8) Windows and buffered UARTS 9) Serial Port configuration 10) Notebook PCs 11) Supplier information 12) About 'Cix' INTRODUCTION ------------ Fast serial cards have become very popular lately due to the proliferation of high speed (V32/V32bis) modems. Whilst the computer press have been keen to cover modems in general, little coverage has been given to serial cards (they're just not sexy enough for mainstream articles). This means that, whilst many people want or need fast serial cards, there is much confusion about them. This document attempts to answer the most common questions asked. 1) What is a UART ? ---------------- Throughout this document, we will refer to something called a 'UART' (pronounced 'you-art'). As this is a very important component, it is important to understand what it is. A UART is an IC (chip!) which takes parallel data from the PC and outputs it to the serial port as a timed serial stream of data. It is the main component of the serial card; all other components are there to support the UART. Here is a simple functional diagram: PC's BUS Slot __________ Serial port 1 ------------> | | 1 ------------> | UART | 0 ------------> | | 1 ------------> | | ---------> 11010001 0 ------------> | | 0 ------------> | | 0 ------------> | | 1 ------------> |________| The diagram shows conversion of data from a parallel to a serial stream. Of course, serial ports can receive data as well as transmit it, so the UART also converts serial data back into parallel data. 2) Why the sudden craze for fast serial ports ? -------------------------------------------- Now that high speed modems (V32, V32bis) have become very common, many PC serial ports are having problems running fast enough to keep up with the modems which can be driven at speeds of up to 57600 bps (bits per second). A standard '8250 UART' based serial port (or one using a multi i/o chip) cannot run at that speed due to the load placed on a typical working PC. The '16450' UART is slightly better. 3) Why aren't normal PC ports good enough ? ---------------------------------------- Standard serial ports were designed to drive mice, plotters and modems. At the time, modems ran at speeds of up to 4800 bps, and we thought that was fast - anyone who has been using modems for more than 5 years will remember how lucky we thought we were ! Problems with serial ports arise for two reasons: a) The 8250/16450 or equivalent UARTS are simply not fast enough to run at the high speeds required by fast modems. Whilst the BIOS/UART will allow you to select these speeds, they may not reliably work at these speeds. The chips are rated for high speed use, but that assumes a simple working enviroment (not a PC!). b) PC's are busy things, with lots of interrupts to service. If the port is running at high speed, the PC may miss interrupts generated by incoming data because it is 'busy doing other things' - this then means that the UART has nowhere to put the incoming byte and so it gets overwritten by the next byte received. This usually causes CRC errors. The faster you drive the port, the more CRC errors you will get. This is almost always a problem if you multitask with MS-Windows or Desqview, as the rapid task switching keeps the processor very busy. So, for instance, if you need to run your serial port at 38400 bps or higher under MS-Windows then it is very likely that you will need a buffered serial port. The '16550' UART gets around both of these problems: a) It is designed to run at high speed; a 16550 chip will run happily at 57600 bps or faster which is fast enough for even the fastest modems (for the moment anyway, but V.Fast modems, just around the corner could require speeds up to 115200 bps. A 16550 is rated to run at this speed, but no-one has really done any experiments yet to prove this). b) The 16550 UART incorporates a 16 byte buffer; if the PC isn't ready to received the current byte then the UART can retain it in the buffer until the PC is ready for it. 16 bytes provides a comfortatble time margin to ensure that none of the received data is lost. Many communications packages will empty the FIFO buffer at every interrupt possible, rather than only taking one byte per interrupt. The buffer on the UART is a 'FIFO' buffer. This simply means that the first byte in will be the first byte out. The alternative to this would be a LIFO buffer, but that would be useless for a UART as all the data would become scrambled ! Analogy ------- Imagine a mail man in the US collecting mail in a street. Assume that each mail box can only hold ONE letter and that people raise their 'flag' on their mail box to indicate that they have a letter for collection - the flag is equivalent to a PC's 'interrupt'. The mail man sees the flag and runs along to collect the letter. If there is a certain level of mail being collected, then it all works well - the mailman has enough time to collect all mail (equivalent to servicing all interrupts). However, if someone puts a letter in their box and the mailman is too busy to collect it before that same person has another letter to mail, then there is a problem: As the mail box can only hold one letter, they would have to destroy the first letter. This is how a normal serial port works. A buffered serial port is simply a bigger mailbox, which can hold up to 16 'letters' at once, so that when the mailman comes, he can collect up to 16 letters at once. ( NOTE : The author of this document is in England, where we don't have mailmen who collect post from our houses - we have postmen who deliver post and red pillar boxes into which we deposit our post - the system referred to above is the one used in the US, because it wouldn't work with the UK system. Any non-US people unfamiliar with these 'flags' should refer to a 'Peanuts/Charlie Brown' cartoon where they seem to feature regularly.) 3a) Internal (PC Card) Modems ------------------------- Some people have been confused about differences between internal (PC Card) and external (desktop) modems. An internal modem plugs into a normal expansion slot inside a PC. Instead of plugging into the PC's serial port, it has its own serial port on board and so takes its data directly from the PC BUS. BUT REMEMBER : An internal modem still has a UART which, just like with a normal port, can be an 8250, 16450 or 16550 (or circuitry which behaves like one). An interal card modem can either have a physical 40-pin UART or sometimes it is embedded within another controller so you may not be able to spot it by looking at the board. When buying a V32bis (or faster) internal card modem you should insist that it has either a true 16550 or 16550 'functionality'. You can test this using the '16550.EXE' program or if you have MS-Windows, the MSD program will tell you which type of UART the modem has. This can of course be used to check the UARTs of regular serial cards too. External modems do not need to have buffered UARTS within the actual modem - the buffered UART is only required within the PC (either on a serial card or a modem card) because the flow control system used across the PC BUS is inadequate for modems (the interrupt method) - once the data is in 'serial form' then flow control becomes much more reliable. 4) How would I know if I needed a faster serial card ? --------------------------------------------------- If you are multitasking using MS-Windows or Desqview, then you will almost certainly need a fast buffered serial card. Typical symptoms will be CRC errors or block retries reported when downloading files. As you will probably be using error correction, then these errors are unlikely to be caused by anything else. Another symptom of an inadequate serial port is that you're simply not getting the performance you'd expect from your modem. This could be caused by you not having your modem configured correctly, so in the first instance, check with your suppliers or colleagues/friends for any special setup strings particular to your modem (perhaps you have forgotten to enable RTS/CTS handshaking or data compression). If you have your modem set up correctly and your performance still isn't what it ought to be, then a faster serial card may be the solution. On a V32bis/V42bis modem, you should be able to achieve download speeds of up to 3000 cps on ASCII text, or 1500 cps on compressed (e.g. ZIP) files. Remember that poor phone lines can also affect modem performance. Even if you are not using Desqview or MS-Windows, you may still get the symptoms details above, especially if you are driving the modem at 38400 bps or faster, in which case you would be wise to invest in a faster serial card. Having said that, there are some people using high speed modems whose existing serial port may be adequate, in which case there is no point investing in a faster one ! A fast serial card with a 16550 UART can be checked for using a number of commonly available utilities. If you have MS-WINDOWS 3.1 installed, then from DOS, you can run the MSD program which you'll find in your C:\WINDOWS directory. Under the option labelled 'Serial Ports' you can view your existing ports and MSD will report the UART type fitted (8250, 16450 or 16550). 5) Okay, I want one, what should I buy ? ------------------------------------- The important part about a fast serial card is its UART; this means that you may be able to simply swap the UART of your existing serial card, rather than having to fit a whole new card. You can only do this if your current serial card uses a 40 pin UART (8250, 16450 or equivalent). This needs to be socketed for easy removal, unless you're particularly experienced with a soldering iron and fancy desoldering the chip yourself - this is detailed in section (7). Unfortunately, most PC's nowadays seem to come with multi i/o cards with one multi-purpose chip which performs serial, parallel and disk operations; if you have a multi i/o card, it will normally be a PLCC package in the centre of the board ( a flat 'surface mounted' chip with pins on all four sides). This obviously cannot be replaced with a 16550. Some PC's have the i/o circuitry built into the motherboard; it is unlikely that you can replace the UARTs there. Thankfully, and more easily, you can simply buy a new card based around a 16550 UART. This can either replace or add to your existing serial ports and your supplier will advise you on the best type. NOTE : This document relates exclusively to buffered serial cards based around the 16550 and equivalent chips. There is another kind of high speed serial card which uses an on board processor; these are generally considerably more expensive and although there are some circumstances where they might be of use, for the vast majority of users, a 16550 based board should be adequate for speeds up to 57600 bps. A well known 'smart' serial board is the 'ESP' which is made by modem manufacturer 'Hayes'. 6) Is there just one type of 16550 UART ? -------------------------------------- No! As with all semiconductors, many manufacturers have jumped on the 16550 bandwaggon. This is good inasmuch that there is a choice, but bad in that some manufacturers have got it wrong - some have FIFO buffers which don't work and some aren't recognised correctly by applications (this can sometimes be worked around but ideally get one which you know will work). The 16550 chip 'by which others are gauged' is made by National Semiconductor, and the original part no. was NS16550AFN. However, due to the increasing popularity of the chip, NS decided to change the name. The latest version is now the PC16550DN. There were earlier version of the original 16550 which didn't work correctly, but it is highly unlikely that any of these are still around (it was the NS16550N - i.e. not the 'AN' or 'AFN'). Other manufacturers have now brought out their own 100% compatible versions of the NS UART, so you may find that your supplier stocks a different version, some of which work well, others don't. As long as he guarantees 100% compatibility it should work as well as the NS original. Many 'clones' are made in what's known as 'CMOS' versions. They are functionally identical (CMOS chips users less power) so they should work fine too. A CMOS UART will normally be called a '16C550' or similar. Being the original, the NS type is the most expensive 16550 UART, so you will probably save money by buying a clone, but take note of the above guidelines. The FIFO buffer on a 16550 chip can be enabled and disabled. By default it will be DISABLED. Many comms programs will automatically detect and enable the FIFO buffer, but if yours doesn't, there are several programs to enable them, which you can include in your AUTOEXEC.BAT file or another batch file. Ask your supplier. If you are running under MS-Windows then see section 8. There are several programs around to test 16550 chips, once they are installed so you'll soon know if your new chip/card is okay. If you can't get hold of one, ask around on BBS's or ask your supplier. MSD.EXE is supplied with Windows 3.1 and will detect/report 16550 UARTS, but there are a couple of problems with MSD; see Section 8. 7) Replacing a soldered 8250 or 16450 UART --------------------------------------- If your existing UART is a soldered 16450 or 8250 type (40 pin DIP) and you are competant with a soldering iron, then you may want to remove the chip yourself and replace it with a 40 pin socket (into which you plug your new 16550). It is not too complicated - the most important thing is to work slowly and carefully. Here are some useful tips from Bob Kellock: >>>> ALWAYS OBSERVE STATIC PRECAUTIONS WHEN HANDLING CHIPS <<<< Get hold of a scrap double sided printed circuit board to practice on; it's no use practicing on a single sided board as they're easy ! >>>>>> DO NOT ATTEMPT TO REMOVE THE CHIP WHOLE ! <<<<<< Get hold of some fine nosed wire cutters and chop off each of the legs of the UART as close to the body as possible (therefore leaving as much of the leg on the board as possible). Discard the main body. Then, remove each of the legs in turn using a soldering iron and tweezers. (Editors note: Bob says "real men use their fingers", but I'd still recommend tweezers). Don't 'wiggle' the iron about on the surface of the board as you will probably damage the tracks/pads by doing so. The time between applying the iron and pulling the leg is fairly critical - just keep practicing. Make sure you pull the leg out square from the board or you may damage the tracks/pads on the component side. You may find that it helps to tin the iron with new solder now and then, but don't use too much otherwise you may end up with a pubble of solder, which will be a read problem. Get hold of some solder wick or a solder sucker to remove the solder from the vias (holes in the board where the legs came from). As a last resort you can use stranded wire and Fluxite but it's very tedious. You may find that some holes just don't want to clean out - resolder them with fresh solder and try again. Check that the holes really are clean - they can be deceptive. Hold the board up to the light and check that you can see through each clearly. Clean off at least the component side of the board with PCB cleaner or meths; use it in a well ventilated room using a stiff brush - an old toothbrush is ideal. Carefully inspect the component side of the board - preferably with a magnifying glass. You can get minute traces of solder between pads which completely messed it up. Remove any odd bits with a scalpel. Check that there is plating through the holes - it is possible to pull it out with the leg but it's not too terrible. Insert the socket in the board with the notch in the right direction. This is when you may find out that one (or more!) of the holes isn't cleaned out properly. Do it again. Solder socket in position, clean, inspect and rectify as above. If you lost any of the through hole platings then solder those pins on the top side of the board too. The board is now ready to take your new 16550 chip. Remember to take static precautions (earth yourself and don't rub your slippers on the nylon carpet). Ensure that all pins on the 16550 are straight; if they are not, carefully press the chip on a table at a right angle to straighten them. Pins tend of break if they are bent too much, so be careful. Place one side of the chip just into the socket; do not push it in yet. Then, place the other side's pins just into the socket. Check that all pins are in the right holes and that they are all straight. Slowly press the chip into the socket using even pressure, making sure no pins snag on their way in. Once it's in. Give it a firm push against the table to make sure everything is home tightly. Check that the chip is inserted properly, the right way round and that none of the pins have curled up under the body instead of being in its correct hole. 8) MS-WINDOWS and high speed UARTS ------------------------------- Microsoft Windows is a prime cause of lost interrupts. This is mostly due to the fact that MS-Windows is processor intensive; as such, with high speed modems, it is even more likely that data will come in quicker than the PC can service the interrupts. This is made worse by the fact the Windows Multi-tasks. A properly configured 16550 UART should solve these problems; MS-Windows is 16550 aware, though you may need to make a couple of modification to your SYSTEM.INI file to enable the 16550, otherwise it will run in '16450 compatible mode' and you'll get no benefit. Once you have installed your 16550 UART or 16550 equipped serial card, the Microsoft MSD program can be used to check it. MSD is a DOS program which you'll find in your WINDOWS directory; it must be run from the DOS prompt, not from within Windows itself. Remember that if you installed your serial card to use a non-standard IRQ, you set it up with the 'Ports' option within Windows Control Panel. If you look at your serial ports, it will show the type of UART each is fitted with. If you have selected non-standard IRQs or you have a COM4 but no COM3, MSD gets confused, so if it doesn't display the expected results, don't panic ! Next, you'll need to edit your SYSTEM.INI file. You can do this from DOS using any suitable text editor or from within Windows Program Manager, select 'FILE' then 'RUN' and enter SYSTEM.INI as the filename. Windows' text editor will load your SYSTEM.INI file. Find the section within SYSTEM.INI labelled '[386Enhanced]' and check for a line which reads: COMxFIFO=1 where 'n' will be the COM port with your 16550 fitted; chances are that line won't exist, in which case you should add it. If there are other lines beggining with COMx... then it makes sense to add the new line near them. For instance, if your 16550 UART was fitted to COM3, you would add the line: COM3FIFO=1 There are no other options other than '1' here as it is a boolean setting (either False=0 or True=1). When using Windows, there is no need to enable the 16550 UART buffers manually with any programs such as 16550.EXE. In fact, with Windows 3.1, if you enable the UART's FIFO buffers (using a 16550.EXE or whatever) before running Windows, then it won't recognise the COM port any more. A company call CyberSoft, in Australia have produced a replacement comms driver for MS-Windows 3.1 which claims to improve Windows comms performance and make best use of the 16550. The driver may be used for free in non-commercial enviroments. Contact Cybersoft on email at : 100033.1723@compuserve.com. Also, Delrina, the company who make the 'Winfax' software have their own replacement comms driver called WFXDRV.DRV (or similar) which also claims to improved Windows comms performance. WFXDRV is available from many bulletin boards as well as Cix and Compuserve. DOS Applications Under Windows ------------------------------ A slight bug with Windows 3.1 is that if you enable your 16550's FIFO buffers before running Windows, then the port will be unavailable, so make sure you don't enable the FIFOs before you run Windows ! Any DOS comms program should be able to take advantage of the 16550, but it depends on the program so results may vary widely from user to user. A company called Cherry Hill Software have produced a replacement comm driver for Windows 3.1 called CHCOMB.386 which will enable DOS programs running under windows to recognise 16550's properly. It may not be necessary, so try your comms with the regular Windows driver first. It is added to your WIN.INI file. It is almost freeware - the author expects either a US$10 donation or a tacky postcard for his collection. They can reached by email at 76050.2204@compuserve.com. The following lines should also be amended in SYSTEM.INI: device=*combuff (delete this line - its in your WIN.INI) device=chcomb.386 [386Enh] COM3FIFO=1 COM3Buffer=10000 COMBoostTime=4 (normally 2) COMIrqSharing=true Experimentation may be required and doubtless there are people more experienced than me on Windows and 16550's so if you have something to add, please email me and I'll amend this document appropriately. Also, practical experience from other users may help your a lot, so always ask around to see if someone had (and has) solved your problem already. 9) Serial Port Configuration ------------------------- This section is relevant to all serial cards, whether they are 16550 based or the normal 8250/16450 types. There are only two IRQ's officially allocated for serial ports, namely IRQs 3 and 4. In theory, this means that you can therefore only have a maximum of two serial ports. You can have more (i.e. COM3 and COM4) by sharing the interrupts (although different addresses), but this causes problems for modems as you cannot use the ports simultaneously when sharing interrupts. Some serial cards allow you to select non-standard IRQ's, typically any of 2,3,4,5,7 or 9. You can only use non-standard IRQs if your software allows you to and also if that IRQ setting doesn't clash with another i/o card already installed. IRQs 5 and 7 are allocated for the parallel ports, but often not used, so they may be available. Higher IRQ's are only available on 16-bit cards as the IRQ hardware lines are on the second part of the BUS. Typically, if you are adding a third serial port to your PC, and you can't use non-standard IRQs, set your new card as COM3/IRQ4 and don't use your existing COM1 as it shares interrupts - as long as there's nothing connected to a port, it shouldn't generate interrupts. If you need to use your existing COM1, you can put your new card in as COM4/IRQ3, but then you can't use your COM2 as it shares its interrupt with COM4. Always experiment though; an interrupt clash will at worst prevent your machine booting (temporarily!). If all else fails, disable your existing COM1/2 (there should be some jumpers on the i/o card) and put your new card in as COM1/2. Windows Users : If you fit your third port as COM4, the MSD program will report it as COM3 due to the way DOS stores the addresses for COM ports. When MSD reports 'COM3' what it actually means is 'your third serial port'. Other programs may report similarly. 10) Notebook PCs ---------------- Users of Notebook PCs often want to upgrade their serial ports to buffers types. The most common answer is that you can't; to save space, notebooks use multi-function chips, so the chip which does the serial i/o will have many other function too and so isn't replaceable. Worse still, notebooks can't take normal PC expansion cards so you can add additional ports, unless your notebook has a docking station option which can take cards. Also, many Notebook PCs have dedicated modems which fit into propritary slots; again the UARTs on these modems cannot be changed, though any sensible V32bis modem designer would have included 16550 functionalty in his modem design. The only solution to a notebook PC without buffered serial ports is to fit a PCMCIA modem if you have a PCMCIA slot. As well as being very neat and compact, many V32bis PCMCIA modems include 16550 functionality - to check the Microsoft MSD utility or any other 16550 checker should confirm the presence of it. 11) Supplier information -------------------- SEG Communications, based in London, England, stock a range of 16550 UARTs and 16550 based serial cards at very competitive prices. SEG also sell a complete range of modems from all leading manufacturers at highly competive prices. SEG specialises in modems, so you can be assured of excellent service. For full details and pricing information contact: SEG Communications 137 Hale Lane Edgware Middlesex HA8 9QP England, U.K. Tel: (081) 959 3377 Fax: (081) 959 2137 Email: segcom@cix.compulink.co.uk From outside the UK, Fax : + 44 81 959 2137 Popular Model : SC5500 Serial Card; Single port card. COM 1,2,3,4 Selectable IRQ 3,4,5,7,9 Credit Card orders are welcome. Trade enquiries welcome. 12) About Cix ------------- Why are we mentioning Cix here ? Because it is my 'Home' system (the one I use all day) and I am happy to recommend it to everyone; they are not paying for this mention, it's a genuine endorsement :-) Cix (pronounced 'Kicks') is a conferencing based host system based in London, England. It is similar is style to Compuserve although considerably less expensive than other similar services and as it's UK based it has a much more friendly 'local' feel than Compuserve. Services offered include Email, Internet access, local conferencing, Usenet conferencing, Fax sending, gigabytes of file downloads as well as the collective wealth of the 1000's of members in 1000's of conferences, each one dedicated to a particular subject. Details of Cix are available from them on (081) 390 8446 (voice) or their data number on (081) 399 5252. Data speeds from 300 to 28800bps are supported with MNP5/V42bis. You can also telnet into Cix at their address : Telnet cix.compuserve.co.uk ------------------ End of Document ---------------- (c) 1993/4 Michael Spalter, SEG Communications This document may only be distributed un-modified. Reproduction as part of any commercial transaction or distribution via any non-electronic medium is prohibited in part of full without written permission from the author. This file may not be compiled or included within any other document or help system without prior written permission from the author. ----- end of file -----