COURIER HIGH SPEED MODEMS
CHAPTER 7. SYNCHRONOUS OPERATIONS
OVERVIEW
Synchronous mode is required for users
who need to call, or receive calls from,
a Host computer of a large network. The
Host is usually a mainframe. The user's
device is often called a terminal, even
if it is a powerful personal computer.
NOTE: The term DTE in the following
discussion means Data Terminal
Equipment, the end-to-end equipment
involved in data communications. DTE
denotes your terminal or computer and
the remote computer.
Data Rate Synchronization
During synchronous operations, transmit
and receive clocks at both ends of the
phone link control the precise timing of
the data flow. The communications
equipment at the remote DTE (the modem
and network software) and your modem and
DTE must all handle the data at the same
speed.
Protocol Compatibility
The devices at both ends of the link
must also use the same protocol. These
devices are exclusive of the Courier
modem, which is transparent during
synchronous communications. The
Courier's V.42 and MNP error control
capabilities are not used.
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COURIER HIGH SPEED MODEMS
The link protocol may be SDLC
(Synchronous Data Link Control), HDLC
(High-Level Data Link Control), BISYNC
(Binary Synchronous Control) or another
protocol determined by the mainframe
manufacturer. These protocols format
the data into blocks or frames, add
control information, and usually provide
error control.
REQUIREMENTS
Courier modems in asynchronous mode
adapt to many conditions of remote
asynchronous modems. But synchronous
connections to a mainframe require
strict adherence to specific operating
parameters. If you are operating a
terminal (not a personal computer)
designed for a particular network, you
probably need only set the Courier
properly before calling or
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answering. If you have a personal
computer, however, you must find out
what hardware and software you need
before setting the Courier for
synchronous calls.
What You Need to Know
The network's communications management
staff can supply information on the
following requirements:
ù The protocol needed for your calls
ù The software support needed to log
into the network, for example, a
specific communications package or
interface board
ù The phone number, if you will be
calling instead of answering
ù Whether you need to change the
duration of the modem's Request to
Send/Clear to Send (RTS/CTS) delay.
Once the Courier establishes a
synchronous connection, it waits 10
milliseconds (the default value)
after receiving an RTS signal from
the DTE before responding with a
CTS signal.
If it's necessary to change the
duration of the RTS/CTS delay, you
need to reset Register S26. See
the S-Register Summary in Appendix
B.
ù Any restrictions about when you can
call
The RS-232 Interface
Transmit and Receive synchronous timing
pins are required at the RS-232
interface. You'll need either pin 15 or
Synchronous Operations 7-3
COURIER HIGH SPEED MODEMS
pin 24 for Transmitter timing signals,
depending on whether the modem (pin 15)
or the DTE (pin 24) generates the
signals. You'll also need pin 17, for
Receiver timing signals. If you're
using an interface adapter card,
described below, the adapter will
implement these pins. If you're
building your own cable, review the RS-
232 Interface, in Appendix B.
The Synchronous Adapter Card
As a rule, personal computers do not
support synchronous communications. You
will probably have to purchase and
install
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COURIER HIGH SPEED MODEMS
a synchronous adapter card. These cards
are multifunction boards that provide
the following functions:
ù A synchronous port from the DTE to
the modem.
ù One or more synchronous protocols.
Be sure to find out which protocol
the Host mainframe requires before
you purchase an adapter.
ù Additional software functions. For
example, you need to identify the
type of computer or terminal you
are using to the mainframe
software. You will most likely
also have to specify your
application, that is, identify the
mainframe resources you want to
use.
SETTING THE MODEM
Once the modem is in synchronous mode it
no longer recognizes commands. Before
attempting to connect in a synchronous
network, use the following instructions
to configure the modem while it is still
in asynchronous mode.
NOTE: Be sure that DIP switch 1 is UP
(factory setting). The Data Terminal
Ready (DTR) override must be OFF so that
the Courier detects when the DTE raises
and lowers the DTR signal.
To set the modem, follow the
instructions for issuing commands to the
modem that are in Chapter 2, Testing the
Installation. Commands begin with a
required AT prefix and end with a
required Carriage Return, which we
denote with the symbol <Enter>. For
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example, the following command sets the
modem to synchronous mode:
AT &M1 <Enter>
Modulation/Link Rate (Bn, &Nn)
Use the following guidelines for your
Courier type.
V.32 bis modems: If the modem is
connecting with another USRobotics V.32
bis modem, set both modems to B0 and to
a variable link rate, &N0. The modems
will connect at the highest possible
rate.
If the Courier V.32 bis is to connect
with a V.32 modem, set the Courier to B0
and try a variable link rate first, &N0.
If that
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doesn't work, try a fixed link rate of
&N6 (9600 bps) or &N3 (2400 bps).
HST modems: We do not recommend
synchronous communications for these
modems because of their asymmetrical
modulation, unless you are connecting
with another USRobotics HST modem. In
that instance, set both modems to B1.
For high speed connections with another
HST modem, set both modems to a fixed
link rate, &N6 (9600 bps) or &N8 (14.4K
bps). Find out first, however, if the
Mainframe accepts the 14.4K link rate.
For 2400-bps connections and lower, set
both modems to a variable link rate,
&N0.
Dual Standard modems: Set the Dual
Standard modem to B0. Try a variable
link rate setting (&N0) first. If that
doesn't work, you may have to set a
fixed link rate, for example, &N6 (9600
bps).
Flow Control
Only the following Courier settings are
allowed: &H0 or &H1, &I0, &R0 or &R1.
See Chapter 4 for more information on
flow control settings.
Transmit Clock Source (&Xn)
This setting specifies whether the
Courier or your DTE generates the
Transmit clock timing signals for a
synchronous call. Most users will
require the default setting, &X0.
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&X0 The Courier is the source of the
Transmit clock timing signals and
sends them to your DTE over the RS-
232 interface. Default.
&X1 The DTE is the source of the
Transmit clock timing signals and
sends them to the Courier over the
RS-232 interface. This setting is
used typically in leased line
multiplexed operations.
(Multiplexors divide the phone
channel so that the channel carries
several calls at the same time.) We
do not recommend this setting for
HST modems.
&X2 The Courier's Receiver clock is the
source of the timing signals. The
signals are looped to the Transmit
clock and sent to your DTE over the
RS-232 interface. This setting is
only used in those systems that
require synchronization of
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COURIER HIGH SPEED MODEMS
data flowing in both directions. We
do not recommend this setting for
HST modems.
Synchronous Mode (&M1)
The modem must be set to &M1 for
synchronous operation. The modem
remains in asynchronous command mode
until it makes a synchronous connection
with a remote modem. Upon connection,
the Courier enters synchronous mode and
sends synchronous timing signals to your
DTE. Note that the Courier does not
respond to commands until it returns to
asynchronous Command mode.
DIALING OUT
Be sure you have set the modem correctly
for all of the functions just described
under Setting the Modem, including the
&M1 setting. The following is a sample
command:
AT &N6 &M1 <Enter>
To call a synchronous Host computer, do
the following:
1. Be sure the modem is assembled
according to the directions in
Chapter 2: a) using the bottom label
as a guide, plug one end of the phone
cord that came with the modem into
the modem jack indicated by the wall
jack icon; b) plug the other end of
the phone cord into your wall jack;
c) plug your phone's cord into the
modem's second jack, indicated by the
phone icon.
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COURIER HIGH SPEED MODEMS
2. Pick up your phone handset and dial
the remote system. As soon as you
finish dialing, press in the
Voice/Data switch. The modem goes
off hook in Originate mode and makes
the connection. You can hang up your
phone.
NOTE: If you've changed the setting of
the Voice/Data switch, insert S32=1 in
the AT . . . &M1 command string
described above. This resets the
switch's function, so that when you
press the switch, the modem goes off
hook in Originate mode.
ANSWERING
You have two options, listed on the next
page:
Synchronous Answer Mode
1. Before powering on the modem, set DIP
switch 5 UP to enable Auto Answer.
Then power on the modem and follow
the previous instructions under
Setting the Modem.
OR
2. When the modem is operating with DIP
switch 5 DOWN, Auto Answer
suppressed, follow the previous
instructions under Setting the Modem,
but insert S0=1, to enable Auto
Answer, in the AT . . . &M1 command
string.
The modem responds automatically to an
incoming call, enters synchronous mode
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and, if set to &X0, begins sending
synchronous timing signals to your DTE.
HANGING UP
The modem remains online until the
remote modem disconnects or your
software causes the DTE to drop DTR.
The Courier sends the NO CARRIER result
code if result codes are enabled, and
returns to asynchronous Command mode.
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