Courier 56K Business Modem Command Reference

Flow Control

This chapter contains information about:

Hardware and Software Flow Control
Received Data Flow Control
Transmit Data Flow Control

Overview

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 control’s 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	Command
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. Note: This is not relevant for typical PC users.	AT&R0
Ignore the RTS signal. &R1 is required if your computer or software does not support RTS	AT&R1
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.	AT&R2
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.	AT&I0 (Default)
Enable software (XON/XOFF) flow control. Use in ARQ mode only. 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).	AT&I1
Force the modem to act on your XON/XOFF commands, but remove them from the data stream instead of passing them to the remote computer. 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.	AT&I2
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 local’s 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.	AT&I3
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.	AT&I4
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 device’s buffer from overflowing. At the computer/device interfaces, the devices independently control the flow of data through their Transmit Data (AT&H) settings.	AT&I5

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	Command
Disable transmit data flow control	AT&H0
Enable Hardware flow control. Requires that your computer and software support Clear to Send (CTS) at the EIA-232 interface.	AT&H1
Enable Software flow control. Requires that your software support XON/XOFF signaling.	AT&H2
Use both hardware and software flow control. If you are unsure about what your equipment supports, select this option.	AT&H3

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Appendixes