ATT Passageway For Partner Communications System User Guide

							Appendix A: Menu Trees
Figure A-3. AT&TSet Menu Tree
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							Appendix A: Menu Trees
Figure A-4. Log Viewer Menu Tree
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							Appendix A: Menu Trees
Figure A-5. AT&TConnect Menu Tree
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							Appendix B: PC Serial Ports
Overview
This appendix provides detailed information about PC serial ports, including
background information about what they are and how they work, how Microsoft
Windows 3.1 manages them, how to resolve problems using COM3 or COM4
under Windows, and how to choose serial port hardware that is well-suited to
PassageWay Solution. If you are familiar with serial port terminology (for
example, I/O port addresses, IRQs, etc.), you may wish to skip over the
background section. If not, you should review the background section before
reading further.
If you are having problems using PassageWay Solution on COM3 or COM4,
you should refer to Workarounds and Solutions to the IRQ Conflict Problem
for advice on solutions and workarounds. If you intend to purchase an add-on
serial port card for use with PassageWay Solution, you should refer to
Selecting an Add-In Serial Port Card for information that can assist you in
selecting a card.
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							Appendix B: PC Serial Ports
Background
Serial ports (also sometimes referred to as communications ports or COM
ports) are hardware interfaces which permit your PC’s microprocessor to
communicate with peripheral devices using a communications standard called
RS-232 (hence, serial ports are also sometimes referred to as RS-232 ports).
Many common computer accessories make use of serial ports, including serial
mice, modems, serial printers, and the PassageWay adapter.
Under DOS (and Windows, which works cooperatively with DOS), the serial
port interfaces in a PC are uniquely identified by specific device names:
COM1 (serial communications port 1), COM2 (serial communications port
2), and so on, usually up through COM4. A particular PC might have none of
these devices, some of them, or all of them installed. For example, most PCs
currently on the market arrive from the manufacturer with two serial ports
already installed (COM1 and COM2), often integrated onto the computers
main system board. Installing additional serial ports (for example, COM3 or
COM4) is usually accomplished by purchasing an add-on card and installing it
into a free expansion slot.
For most purposes (for example, configuring software), the generic description
of serial ports provided by their device names is sufficient. For example,
during the PassageWay Solution installation procedure, you are asked to
provide the device name of the serial port to which you have connected the
PassageWay adapter (for example, COM2). Unfortunately, this abstract view
of serial ports is not sufficient for other purposes, notably for troubleshooting
problems: to be able to do this effectively, a basic understanding of serial port
hardware is required. In particular, it’s essential to understand the mechanics
by which the computer’s microprocessor communicates with serial port
hardware.
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The microprocessor/seriai port communication consists of two aspects: an I/O
port address and an interrupt request signal (IRQ). The I/O port address 
						

							Appendix B: PC Serial Ports
represents a small region of the microprocessor’s input/output memory space
which is used to pass data back and forth to the serial port. This memory
region acts something like a mailbox: outgoing mail (data from the
microprocessor to be transmitted to the peripheral device) is placed in the
mailbox by the owner (the microprocessor) to be picked up by the mail carrier
(the serial port hardware) for subsequent delivery to the destination party (the
peripheral device). In turn, the mail carrier (the serial port hardware) places
incoming mail (data from the peripheral device) into the box to be picked up by
the owner (the microprocessor). This analogy illustrates an additional
important point about I/O port addresses: just as individual mailboxes help the
residents in a neighborhood keep their mail from getting mixed up, each device
using an I/O port address to communicate with the microprocessor should
have a unique address that doesn’t conflict with that of any other device.
The mailbox analogy is also helpful in understanding the IRQ mechanism.
Normally, we place our outgoing mail in our mailbox at any convenient time
before the mail carrier arrives to pick it up. The outgoing mail sits in our
mailbox until the mail carrier arrives, at which time it is picked up and possibly
some incoming mail is placed in the mailbox. Then, sometime later, we check
our mailbox and retrieve our new incoming mail. The problem with this
scheme is that it’s not very efficient both the outgoing and incoming mail
spends some time just sitting in the mailbox. A better approach would be if the
mail carrier provided some sort of signal (for example, ringing the doorbell) to
announce his or her arrival, in which case we could hand over the outgoing
mail and pick up the incoming mail immediately. ln the PC architecture, IRQs
acts like the doorbell in our analogy they provide a method by which
hardware devices in the computer can get the microprocessor’s attention to
efficiently deal with some process. The serial port hardware makes use of an
IRQ to announce that it’s ready to receive more outgoing data and/or that new
data has arrived from the peripheral device which needs to be processed.
Like I/O port addresses, IRQs must generally be unique among the active
hardware devices in a computer system. In the mailbox analogy, the doorbell
is probably not a good signal since virtually anyone could ring the doorbell for
any number of reasons, not just to indicate the arrival of mail. Similarly, if a
particular IRQ signal is used (PCs generally support 16 unique IRQ signals,
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							Appendix B: PC Serial Ports
denoted IRQ0, IRQ1, and so on, up through IRQ15), the microprocessor must
take the appropriate action for the device associated with that IRQ. If there is
a mix-up, or if more than one device attempts to use the same IRQ at the
same time, a conflict occurs, and the outcome is often unpredictable and
usually undesirable (for example, the computer may hang). Because IRQs
are a limited resource, some newer PCs support IRQ sharing, a hardware
mechanism which permits more than one device to make use of the same
IRQ, but most PCs do not. For example, all PCs which use IBM’s
MicroChannel Architecture (MCA) support IRQ sharing, as do most PCs which
use the Enhanced Industry Standard Architecture (EISA) design. However,
most PCs in the marketplace - even new models - are based on the
traditional Industry Standard Architecture (ISA) which generally does not
support IRQ sharing.
The specific I/O port address and IRQ that a particular serial port uses is
determined by the hardware configuration of the serial port. Generally, these
parameters cannot be changed for built-in serial ports, but add-on cards
containing serial ports often provide jumpers or switches which can be used to
configure them to use one of several I/O port addresses and IRQ
combinations. The table below lists the default I/O port addresses and IRQs
used by the serial ports of IBM PC/AT-compatible computers:
Serial Portl/O Port AddressIRQ
Device Name
COM103F84
COM2
02F83
COM3
03E84
COM402E83
The values in this table play an important part in understanding the wrinkles
associated with serial ports: although there is provision for up to four serial
ports, with four unique l/O addresses, there are only two unique IRQs
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							Appendix B: PC Serial Ports
associated with them (recall that most PCs require the IRQs used by each
active device to be unique to avoid conflicts). To understand why, its useful
to recall what the PC world was like before the widespread availability of
products like Windows. When the architecture of the current generation of
PCs was first being designed (for the IBM PC/AT), the concept of multitasking
was not nearly as important in the PC marketplace as it is today.
Consequently, since DOS (before Windows) did not permit multiple
applications to run simultaneously (with the notable exception of certain
specialized programs such as mouse drivers), there was little need to provide
a mechanism by which several serial ports could be operated simultaneously.
Consequently, the strategy used was to conserve IRQs by assigning the same
IRQ to more than one COM port (that is, the COM1 and COM3 ports were
both assigned to IRQ4, and the COM2 and COM4 ports were both assigned to
IRQ3). Then, under the assumption that at most two serial ports would be
active simultaneously (for example, COM1 and COM2, which have unique
IRQs), conflicts would not occur.
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							Appendix B: PC Serial Ports
Serial Ports Under Windows 3.1
Unlike the DOS-only world of yesterday, today’s multitasking environments like
Windows permit the microprocessor to communicate with up to four active
serial port devices at the same time (COM1 through COM4). For example,
under Windows, if you are using a serial mouse (on COM1 ) within a terminal
emulator program which operates a data modem (on COM2), while using a fax
board (on COM3) to transmit or receive a fax in the background, you are
using three serial port devices simultaneously. You might even wish to make a
phone call using PassageWay Solution (on COM4) at the same time, bringing
the total up to four simultaneously active serial port devices.
The fact that Windows permits this kind of powerful multitasking does not
guarantee that the underlying PC hardware can support this level of operation,
at least without some customizing at the hardware level. Since some PCs can
support it by default (for example, those which support IRQ sharing), Windows
doesn’t prohibit you from configuring your system and attempting tasks like the
one in the previous paragraph. Unfortunately, most PCs cannot support this
operation by default and the most likely result of attempting the above
scenario is hanging the PC due to an IRQ conflict. On such systems, using
COM1 along with COM2 is generally fine (recall that these devices have
unique IRQs by default), but the addition of COM3 or COM4 causes the
system to fail.
Fortunately, Windows 3.1 permits complete customization of all parameters
involving serial ports through the Control Panel, including configuring
nonstandard I/O port addresses and IRQs (that is, values different from those
in the table – these parameters can be viewed and/or modified by selecting
the desired port in the Control Panel’s Ports applet, selecting the Settings...
button, then selecting the Advanced... button). This flexibility offers the
opportunity of salvation for owners of PCs which do not support IRQ sharing
who require the use of three or more COM ports simultaneously.
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							Appendix B: PC Serial Ports
Workarounds and Solutions to the
IRQ Conflict Problem
It is important to remember that the IRQ conflict is a problem in hardware; it
cannot be resolved in software alone. Consequently, there are only three
alternatives for working around or resolving it:
Workaround 1: Configure your serial devices such that you use only two
at any one time, and those two use serial ports with unique IRQs.
This is the simplest workaround to the IRQ conflict problem, but it does not
solve the underlying conflict. The idea is to assign your peripheral devices to
your available serial ports in such a way as to avoid using any devices
simultaneously which might conflict. For example, if you have a serial mouse
on COM1 (IRQ4), a fax/modem card on COM2 (IRQ3), and your PassageWay
adapter on COM3 (IRQ4), you cannot effectively use PassageWay Solution
since you need to use your mouse under Windows while PassageWay
applications are running.
A better arrangement would be to move PassageWay Solution to COM4
(IRQ3), which could then be safely used with your mouse on COM1 (IRQ4). In
this case, the workaround is to avoid trying to use PassageWay Solution at the
same time you use the fax/modem on COM2, since the conflict now would be
over IRQ3 (COM2 and COM4).
Workaround 2: Replace one or more of your serial peripherals with
equivalent devices which do not require a serial port.
The idea with this approach is to eliminate the conflict by reducing the number
of peripherals in your system which require serial ports. For example,
replacing a serial mouse with a bus mouse (that is, a mouse which requires its
own add-in card) would make another serial port available that could then be
used by another device.
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