Dec 062017
A diagnostic program for both serial and parallel ports with the capability of reassigning ports. It is fast, well documented, and easy to use.
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Category System Diagnostics
A diagnostic program for both serial and parallel ports with the capability of reassigning ports. It is fast, well documented, and easy to use.
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PORTTEST.EXE 67968 33942 deflated

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Contents of the PORTTEST.DOC file


Comprehensive I/O Port Diagnostics for PCs


Version 1.1

4100 Moorpark Ave. #104
San Jose, CA 95117 USA
Phone: (408)296-4000
FAX: (408)296-5877

Copyright (C) MicroSystems Development 1990
All rights reserved


MicroSystems Development makes no warranty of any kind, express
or implied, including without limitation, any warranties of
merchantability and/or fitness for any specific application or
use. MicroSystems Development shall not be liable for ANY loss
or damage arising from a failure of this program to operate in
the manner described, or in a manner desired by the user.
MicroSystems Development shall not be liable for any damage to
data or property which may be caused directly or indirectly by
use of the program.



The software code and screen displays used in PORT TEST are the
sole property of MicroSystems Development, and may not be copied,
in any form, in whole or in part, or included in any other
program or document without the express written permission of
MicroSystems Development.

IBM, IBM PC, IBM XT and IBM AT are registered trademarks of
International Business Machines Corporation.

MS-DOS is a trademark of Microsoft Corporation.

PORT TEST is a trademark of MicroSystems Development.

MicroSystems Development is a division of
MicroSystems Development Technologies, Inc.


1.0 BACKGROUND INFORMATION................................1
1.1 Serial Port Basics...............................1
1.2 Parallel Port Basics.............................1

2.0 ADDRESSING SCHEMES....................................3
2.1 Port Addressing..................................3
2.2 What the BIOS does...............................3
2.3 What DOS does....................................4
2.4 Logical vs. Physical Addresses...................4
2.5 Interrupt Usage..................................5

3.0 SERIAL AND PARALLEL TESTS.............................6

4.0 COMMAND LINE PARAMETERS...............................7

CUSTOMER FEEDBACK FORM.....................................8

In addition to tests and diagnostics, PORT TEST can do
the following:

porttest swap lpt1 lpt2
Swaps table addresses for lpt1 and lpt2.

porttest com3:19200,n,8,1
Set baud rate, parity, data bits, and stop bits for a serial

porttest as 280
Adds address 0280 hex to the serial port table as the next
logical com port.

porttest ds
Automatically detects serial ports and updates the table.

And more...


When computer information, known as data, is sent from one
device to another, it is carried by a wire cable. Data is
generally sent in one of two basic ways. Many bits at a time
over many wires in the cable is known as parallel transfer.
Sending one bit at a time over one wire is known as serial
transfer. Each type of transfer has its own advantages and
disadvantages. The serial or parallel connections to the
computer are called PORTS. Collectively, they are called
Input/Output, or I/O ports.

PORT TEST is comprehensive diagnostic program for I/O ports
installed in IBM PC and AT compatibles computers running DOS. It
is useful to identify ports, manipulate system tables, test the
ports, and to identify and correct problems. This manual is
intended to supplement the information provided in PORT TEST by
explaining the ways in which I/O ports are commonly used and
implemented in IBM compatible PCs. It also describes what each
test does and provides information on what to look for if any of
the tests fail.


The PC serial port uses voltages and connectors established
by the Electronic Industries Association (EIA) standard
specification known as RS-232C. In addition to one bit of data,
other control signals are also sent and received. Sending data
one bit at a time is relatively slow, but serial ports have the
advantage of using fewer wires to connect two devices and being
able to communicate over relatively long cable distances. Most
serial ports can easily communicate over 100 ft of cable. Data
is sent using +12 volts and -12 volts to represent a 0 or a 1
data bit. In addition, there are a wide variety of serial
peripherals available which make use of the standard RS-232C
specification. Furthermore, recent advances have made serial
data transmission reliable at higher data rates. These data
rates are referred to as "baud" rates.

The heart of all serial ports is an Asychronous
Communications Element, referred to as an ACE. The CPU reads and
writes a byte (8 bits) of data to this chip, and the ACE handles
the serial transmissions and other functions related to the RS-
232C specification. The ACE used in most PC & XT type (8088 and
8086) systems is the INS8250B or equivalent. Most AT class
machines (80286 & above) use the NS16450 device. The 16450 chip
is designed to handle the higher data bus rates of the faster
computers. The serial data communications however is identical
to the 8250. It is beyond the scope of this manual to describe
the detailed workings of these chips. Refer to the
manufacturer's data sheets for more detailed information.


Eight bits of data are transferred at a time over PC
parallel ports. Data is sent using +5 volts and 0 volts to

1 PORT TEST User's Manual

represent a 1 or a 0 data bit. As you might expect, data can be
transferred relatively fast in this manner. The disadvantages of
this method is that more wires are needed and that most common
parallel ports can only work reliably at distances of under 20

Unlike serial ports which rely on a chip to do the data
transmission, parallel data is handled entirely with software.
Parallel ports have three registers, one for data out, one for
output control lines, and one for input control lines.

The way is which data is transmitted over parallel ports
varies, but a general description follows. To send a byte of
data, the software outputs the byte to the data lines, then
pulses the STROBE output. The device on the other end (normally
a printer) then asserts its BUSY line. The PC waits for BUSY to
go away before sending the next byte. In other implementations,
the ACKNOWLEDGE line is used for the printer to signal to the PC
that it has received the data.

The other control lines are used for various purposes. On
the printer side, PAPER-END is a signal to the PC that the
printer is out of paper. The printer also sends a SELECTED
signal to the PC indicating that it is 'on line' and ready to
receive data. The ERROR signal from the printer can be used for
any type of error that would cause the printer to be unable to
receive data. In general, if any of these signals are asserted,
BUSY is also asserted.

The PC has three additional output control lines. Usage of
these lines varies with the particular software and printer in
use. In the original PC implementation, they are used as
follows: AUTOFEED tells the printer to automatically insert a
line feed after each carriage return. INIT initializes the
printer upon power up. SELECT deselects the printer and takes it
'off line'. SELECT was most often used as a method to signal the
operator that some attention was required. For example, when the
software needed to change to a different font, it would deselect
the printer. This would cause the printer to turn off its
SELECTED line and its front panel light. The user would then
change the print wheel in the printer and press the SELECT button
on the printer. The printer would then turn on its SELECTED line
signalling the software to proceed. Many printers will stay in
the 'off line' condition for as long as SELECT or INIT are
asserted even if the SELECT button is pressed.

As stated earlier, not all of these controls lines are used
in the same manner. Some of the latest laser printers on the
market ignore all the control lines to it except for the STROBE

2 PORT TEST User's Manual



Any I/O device in a PC must be located at a particular
address. Actually, a serial port consists of eight addresses and
a parallel port has three. The address used to refer to a
particular port is the first or BASE address. For both serial
and parallel ports, this first address is where the data to be
transferred is read and written.

These addresses are the PHYSICAL addresses of the ports. It
is where the Central Processing Unit (CPU) of the PC must read or
write data to send or receive data over a port. There are no set
rules for what addresses must be used for ports, but some
commonly used addresses are supported by most manufacturers.
Generally, the first two serial ports are at physical addresses
3F8 and 2F8. These are referred to as COM1 and COM2, the LOGICAL
addresses. Generally, parallel ports are located at addresses
3BC, 378 and 278, which are referred to as LPT1, LPT2, and LPT3.
Many monitor cards have a parallel port built in. In these
cases, the port is usually at 3BC.

NOTE: The physical addresses are shown in hexadecimal notation.


Most PC BIOS routines do a quick check upon power up to
determine how many serial and parallel ports are installed in a
system. They examine specific addresses by writing to them, then
reading them back to determine if a device is present at that
address. The BIOS then places the address of each responding
device in a table in RAM located at 40:0 for serial ports and at
40:8 for parallel ports.

There are eight bytes at each of the above locations
reserved for port addresses. Since a port address is two bytes,
up to four serial and four parallel addresses can be put in the

For example, for a system with two serial ports and two
parallel ports, typical tables might be:

Serial Port table: Parallel port table:
40:0 F8 03 40:8 BC 03
40:2 F8 02 40:A 78 03
40:4 00 00 40:C 00 00
40:6 00 00 40:E 00 00

Note: The high & low bytes of the address are reversed in the

3 PORT TEST User's Manual


After a PC has booted and the BIOS is no longer in control,
DOS uses the addresses in these tables as COM1, COM2, etc. for
serial ports, and LPT1, LPT2, etc. for parallel ports. For
example, DOS includes a program called MODE to set various
parameters such as the baud rate, of a port. In order to use the
MODE command, the user must refer to the desired port by its COM
or LPT assignment. The physical address used is the one stored
in the table. It is very possible that COM1 in one system is not
at the same physical address as COM1 in another system.

In order for DOS to recognize a COM or LPT number, there
must be a valid address listed in the table, and there can be no
gaps. If there is a zero entry in the table, DOS considers that
the end of the list. Therefore, as far as DOS is concerned, it
is impossible to have a COM1 & a COM4 without having a COM2 &
COM3 also! Other software may report differently as explained


Many BIOS routines only check for two serial ports and three
parallel ports. As PCs became more popular, the need for more
ports grew. This fact was 'fixed' by DOS Versions 3.3 and
higher, which will recognize up to four COM and four LPT ports.
However, DOS will only recognize a logical port whose physical
address has been placed in the RAM table. If the BIOS doesn't
put the physical address there upon power up, DOS won't recognize
a logical port even when the hardware is properly configured and

If you need to use DOS to support more ports than your BIOS
will detect and install, PORT TEST can be used to place the
correct address in the system tables. You can use the
interactive mode, but PORT TEST's batch mode was specifically
designed for this purpose.

Place the command: porttest /ds /dp in your autoexec.bat
file to automatically detect serial and detect parallel ports in
your system upon power up. These ports will now be recognized by
DOS. See "Command Line Parameters" for more information.

NOTE: Many application programs do not use DOS to interface
to ports, so that it may not be necessary to include a port's
address in the system tables. However, you must place the
address in the table in order to test a port with PORT TEST.

Considerable confusion exists regarding proper logical and
physical port address. Strictly speaking, there is no proper
physical address for any of the logical ports. The logical port
structure permits any physical address to be placed into its

4 PORT TEST User's Manual

Many application programs access ports directly, and do not
use the built in routines in the hardware's BIOS. This is
usually done because the application needs a more sophisticated
device driver than is provided by the BIOS. When this is the
case, the logical port addresses are not always used properly.
Some applications access ports directly, but properly determine
the physical address from the logical address by using the table
at 40:0. Other programs do not use logical addresses at all, and
just refer to physical addresses. Still others convert logical
to physical addresses using their own table. In this case, the
program could report that there is a COM1 and a COM4 installed
with no COM2 or COM3!


In addition to a serial port's registers, the ACE also has
the ability to interrupt the processor, either after it has
transmitted a byte of data, or after it has received one.
Ideally, the interrupt request line that each port uses should be
unique, but there are not always enough interrupts to go around.
Therefore, interrupts are sometimes used by more than one device.
This is ok as long as only one of the devices is active at a

A standard PC has eight hardware interrupt request lines.
Often, IRQ4 is used for the serial port at address 3F8 and IRQ3
is used for the port at address 2F8. This is what PORT TEST
assumes unless told otherwise. For other serial ports, the user
must assign the proper IRQ number (if any) in PORT TEST. If left
unassigned, PORT TEST will skip the interrupt tests.

Most I/O cards have jumpers to select which interrupt will
be used. If there are conflicts, the user should examine these
jumpers to determine the cause of conflict.

Some parallel ports can be configured to use IRQ7. When
enabled, the processor will receive an interrupt when
/ACKNOWLEDGE (pin 10) transitions from high to low. This is not
supported on all cards and few applications use this feature.
PORT TEST does not test parallel port interrupts.

5 PORT TEST User's Manual


Detailed information on the content of the tests is in the
manual which comes with the registered version of PORT TEST.

PORT TEST User's Manual


In addition to its interactive mode, PORT TEST supports
several command line parameters as follows:

Usage: porttest [mode switches] [batch commands]

All command line parameters are optional.

mode switches:
/p = Use the system's default print screen handler
Normally PORT TEST uses its own print screen handler so that
linedraw and other non printable characters are replaced
before printing. If your printer has the capability to
print these characters, then use this command line option to
turn off PORT TEST's character translation when doing a
Print Screen.

/m = force Monochrome on color monitors

/i force interactive mode
(default when no batch args)

batch commands:

detectserial OR ds -> detect serial ports
detectparallel OR dp -> detect parallel ports
addserial OR as -> add serial port
addparallel OR ap -> add parallel port

,, -> set serial parameters
addserial :,

,, -> add and set parameters
delete com# OR delete lpt# -> delete port
swap com# com# OR swap lpt# lpt# -> swap ports

Examples of command line usage:

porttest ds
Automatically detects serial ports and updates the table.

porttest as 280
Adds address 0280 hex to the serial port table as the next
logical com port.

porttest com3:9600,n,8,1
Set baud rate, parity, data bits, and stop bits for a serial

porttest swap lpt1 lpt2
Swaps table addresses for lpt1 and lpt2.

7 PORT TEST User's Manual


We continue to strive to provide a quality product. Your
comments, suggestions, criticisms, etc. are most welcome. Just
fill out this form and send or fax it to us.

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We guarantee a reply to all registered users who request one.

Mail or FAX this form to: MicroSystems Development
4100 Moorpark Ave. #104
FAX: (408)296-5877 San Jose, CA 95117 USA

8 PORT TEST User's Manual

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