the AT Command Set
Answering, and Hanging Up
Configuring with DIP Switches
Result Code Displays
and Configuring the Courier 56K Business Modem Remotely
Error Control, Data Compression, and Throughput
Querying and Help Screens
Line and Synchronous Applications
Meanings and Sets
V.25 bis Reference
Information and Limited Warranty
Courier 56K Business Modem Command Reference
This chapter contains information about:
The Courier 56K Business Modem has two buffers, one for data transmitted
from your computer, and one for data received from the phone line.
Flow control provides a system for stopping and starting transmission
depending on how full the buffers are. Flow controls purpose is
to prevent overfilling the buffers, which may cause data to be lost.
We recommend that you use hardware flow control. If you do, depending
on your communications software, you will also need to enable hardware
flow control in your communications software.
Hardware and Software Flow Control
There are two kinds of flow control: hardware and software. Courier 56K Business Modems support both, but your computer and communications software must
also support the kind of flow control you choose.
Hardware Flow Control
Courier 56K Business Modems implement hardware flow control by dropping
the Clear to Send (CTS) signal to stop the flow of data when a buffer
is 90% full. When the buffer drops back to 20% full, then CTS is raised
to restart the flow of data.
Software Flow Control
Courier 56K Business Modems implement software flow control by detecting
that a buffer is 90% full and then sending special characters in the data
stream to stop the flow of data. When the buffer drops back to 20% full,
the Courier 56K Business Modem sends special characters in the data stream
to restart the flow of data.
The problem with software flow control is that the characters used to
stop (<Ctrl>Q) and start (<Ctrl>S) the flow of data can occur
naturally in the data flow. Enabling software flow control instructs the
modem to recognise and act on these characters, even if they are not intended
to control the data flow.
Using software flow control may prove satisfactory if you're transferring
text files only.
The start command is called XON (for transmit on) and the stop command
is called XOFF (transmit off). You can change the characters used. Refer
to Registers S22 and S23 in the S-Registers appendix in this guide.
Received Data Flow Control
Flow control settings are controlled by the AT&Rn and AT&In commands.
The default settings are &R2&I0. Use the following table for more
information about setting the flow control.
For your modem to
Pause before sending CTS signal after receiving
the Request to Send (RTS).
The delay is required by some synchronous mainframes and does not
apply to asynchronous calls.
This is not relevant for typical PC users.
|Ignore the RTS signal.
&R1 is required if your computer or software does not support
|Enable hardware flow control.
This sends data to your computer only upon receipt of the RTS signal
and is the normal setting for hardware flow control.
|Disable software (XON/XOFF) flow control.
Recommended for non-ARQ (Normal mode) calls (see AT&I5). While
the Courier 56K Business Modem is online, the only characters it recognises
are +++, the escape code.
|Enable software (XON/XOFF) flow control. Use in ARQ
Note: The XON/XOFF characters
sent to the remote computer may interfere with XON/XOFF signaling
between the remote computer and remote device (see AT&I2).
|Force the modem to act on your XON/XOFF commands,
but remove them from the data stream instead of passing them to the
This ensures that the remote computer does not confuse your XON/XOFF
characters with those from its attached device. This is the recommended
setting for ARQ mode.
|When using the AT&I2 command, if the call is
not in ARQ mode, there is no flow control on the link. If you send
an XOFF to your modem and it stops passing data, it has no way to
tell the remote computer and modem to stop sending for a while, and
the locals buffer may overflow. For more reliable control in
non ARQ mode, see AT&I5.
|Enable Hewlett Packard-Host mode. Applies only to
devices attached to an HP mainframe that uses the ENQ/ACK protocol.
Use in ARQ mode only.
If you want to use software flow control to transfer non-text (binary)
files, set serial port and connection rates equal using &B0 and
& N0. Refer to the Controlling Data Rates chapter in this guide
for more information about these commands.
|Enable Hewlett Packard-Terminal mode. Applies only
to Courier 56K Business Modems attached to terminals in an HP system
that uses the ENQ/ACK protocol. Use in ARQ mode only.
Enable flow control when the connection is not under error control.
For this to work, the remote device must also have AT&I5 capability.
In ARQ mode, a modem set to AT&I5 operates the same as it does
when set to &I2. It acts on your XON/XOFF commands, but does not
pass them to the remote system. The error-control protocol enables
the devices to control the flow of data on the phone link.
In non-ARQ mode, a modem set to AT&I5 operates as though flow
control were disabled (AT&I0); it does not look for your typed
XON/XOFF commands. However, it does look for XON/XOFF characters coming
in over the phone link. When the remote device sends XON/XOFF commands,
the modem either resumes or stops transmitting data over the link
and drops the characters from the data stream.
|If both devices are set to AT&I5, operators at
each end can signal the remote device to stop sending. Thus, controlling
the data flow on the phone link and preventing their own devices
buffer from overflowing. At the computer/device interfaces, the devices
independently control the flow of data through their Transmit Data
Example: Sending AT&I2 will remove XON/XOFF commands from
the data stream instead of sending them to the remote computer. This will
force the Courier 56K Business Modem to act of the XON/XOFF commands.
Transmit Data Flow Control
This type of flow control is for data transmitted to the
Courier 56K Business Modem by its attached computer.
For your modem to
|Disable transmit data flow control
|Enable Hardware flow control.
Requires that your computer and software support Clear to Send (CTS)
at the EIA-232 interface.
|Enable Software flow control.
Requires that your software support XON/XOFF signaling.
|Use both hardware and software flow control.
If you are unsure about what your equipment supports, select this