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Technical Note : FAST SERIAL PORTS Author : Michael Spalter, S/E/G Communications Email : segcom@cix.compulink.co.uk Origin : United Kingdom, EC Last Modified : 13th April 1993 Status : (c) 1993 Michael Spalter NOTE: This document may be distributed freely providing that all credits remain intact and that it is not modified in any way. Any suggestions for improving this document are welcome. Please contact the author as shown at the foot of the document. Bulletin board sysops are welcome to make this file available as a 'Bulletin'. This document is provided for information only and no support or warranty is offered by the author or publisher. ┌────────────────────────────────┐ │ DO I NEED A FAST SERIAL PORT ? │ └────────────────────────────────┘ ┌───────────────────────────────────────────────────┐ │ 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 ? │ │ 4) How would I know if I needs 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) Supplier information │ └───────────────────────────────────────────────────┘ 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 possibly run at that speed. The '16450' UART is only 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. 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 modem. 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 ! 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 ! 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. 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 part no. is NS16550AFN. There is an older NS version, but make sure you order the 'AFN' type. If your supplier stocks a different 16550 version, ensure that it is 100% compatible with the NS16550AFN, and ideally get a guarantee that you can return it if you find that it isn't ! The NS type is the most expensive, 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. 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. Both US Robotics and Hayes make high speed serial cards which are very flexible; ask your supplier for details. If your local supplier cannot get hold of a fast serial card, 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 so 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, but there is a slight complication with Windows 3.1; if you enable the UART's FIFO buffers (using a 16550.EXE or whatever), then Windows won't recognise the COM port any more; this is strange because the MSD program supplied with 3.1 will happily report the presence of a 16550 UART. If your Windows software is '16550 aware' then it will happily get around this, but if you Windows software doesn't, or you are using DOS software under Windows, then you may have to use a replacement COM driver, or use different software. DOS Applications Under Windows ------------------------------ 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 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 WIN.INI: device=*combuff (delete this line - its in your WIN.INI) device=chcomb.386 [386Enh] COM3FIFO=true 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. 9) Supplier information -------------------- S/E/G Communications, based in London, England, stock a range of 16550 UARTs and 16550 based serial cards at very competitive prices. For full details and pricing information contact: ┌────────────────────────────────────────┐ │ │ │ S/E/G Communications │ │ 137 Hale Lane │ │ Edgware │ │ Middlesex HA8 9QP │ │ Tel: (081) 959 3377 │ │ Fax: (081) 959 2137 │ │ Email: segcom@cix.compulink.co.uk │ │ │ └────────────────────────────────────────┘ Credit Card orders are welcome. Trade enquiries welcome. ------------------ End of Document ---------------- (c) 1993 Michael Spalter, S/E/G 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. ----- end of file -----