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COM3COM4.DOC:

Tips on Using COM3 and COM4 Serial Ports

Part I - Port Address Problems

Part II - Converting the IBM Asynchronous Communications  Adapter to use as COM3 or COM4

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Part I - Port Address Problems

SETCOM3 - Set base address of COM3 serial port

  Al Stangenberger  Dept. of Forestry & Resource Mgt.  University of California  Berkeley, CA 94720

  (forags@violet.berkeley.edu)

This discussion and program is adapted from the MS-Kermit User Guide.

Some programs trying to use serial ports COM3 or COM4 will notrun because the base addresses of these serial ports are notstored in the proper low-memory addresses. There are nostandards for the values for these addresses (unlike for COM1 and COM2),hence they may not be in the system ROM Bios boot code which is runwhen the computer is turned on.

This condition may easily be confirmed by attempting to use theMS-DOS MODE command to set parameters for COM3 or COM4. If an otherwisevalid MODE command returns diagnostics such as "Illegal device" or

"Invalid parameters," then these addresses have not been defined yet.

The code in SETCOM3.RAW is a prototype of input to DEBUGwhich can be used to construct a small .COM program tocorrect this problem for serial port COM3. A slightmodification would make it suitable for COM4. Normally,SETCOM3 is only executed once as part of the AUTOEXEC.BATprocedure.

To adapt SETCOM3.RAW to your situation, consult thedocumentation for your serial card or modem to find the baseaddress used when the card or modem is set for COM3.A typical value would be 02E8 (hexadecimal).

Modify lines 4 and 5 in SETCOM3.RAW to have this value.

To construct the program, type the command:

A> DEBUG < SETCOM3.RAW

It's a good idea to use DEBUG to examine the result to besure it is correct. To do this:

A> DEBUG setcom3.com-u  (the assembled code will be listed here)-q

The program is executed by the command

A> SETCOM3

To change from COM3 to COM4, find the base address for COM4 asdescribed above, and change lines 4 and 5 accordingly.Also change line 7 from [4] to [6] to store this address at theproper location in memory. Finally, change the name of the file

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Part II - Converting the IBM Asynchronous Communications  Adapter to use as COM3 or COM4

From: uw-beaver!tektronix!tekgen!ferrouss (Ferrous Steinka)Newsgroups: comp.sys.ibm.pcSubject: Serial ports 3 and 4Date: 25 Mar 88 17:55:48 GMT

So, you've upgraded to a multifunction card and you have an oldIBM asynchronous communications adapter laying around that isjust taking up space. Well, with a simple mod to the board, itcan be made to respond to COM3 or COM4. Of course you will needto have appropriate software to take advantage of your new serialport. IBM MSDOS 3.3 knows about ports 3 and 4, MSDOS 3.1 doesn'tand I'm not sure about 3.2 or 3.21. I tested ProComm and it DOESknow about the secondary ports. The addresses of COM1 and COM2are 3F8(h) and 2F8(h) respectively. The addresses for COM3 andCOM4 are 3E8(h) and 2E8(h) respectively. The difference is addressline A4, and all you have to do is invert it on the board prior tothe decoding logic. This can be done by cutting pin 5 of U-2, a74LS30, at the surface of the board and gently bending it up sothat it may be soldered to. The pad to which pin 5 of U-2 was

soldered is A4, which connects via the edge card connector to thesystem bus. Pin 5 of U-2 is an input to the decoding circuitry. Ichose to use a 74LS04 as the inverter (piggy-backed onto U-3, alsoa 74LS04, to provide power and mechanical stability) . Connect pin3 of the tacked on 74LS04 to A4 and pin 4 of the 74LS04 to pin 5 ofU-2. CAUTION: There are unused buffer/inverters on the board butthey are open collector type devices. Using these could lead totiming problems.

For additional information, consult an IBM Technical ReferenceGuide for either the PC or the XT. Both have schematics of theasynchronous communications adapter.