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Attaching Devices to the Asynchronous Serial Interface

7-4 Peripherals Configuration (R1001H) HP-UX version 11.00.01

Attaching Devices to the Asynchronous

Serial Interface

The following sections describe issues and procedures related to attaching devices

(modems or terminals) to an asynchronous serial interface.

Cable Connection and Flow Control

Cable connection and flow control require some understanding of the concepts

behind them.

DTE and DCE Controllers

An asynchronous port behaves like data terminal equipment (DTE). The

connected device must behave like data communications equipment (DCE).

Historically, a DCE was a classification for modems and DTE was some other

device that terminated the data path, such as a terminal or the computer system

itself. The data flow from one device to another would have been from the system

(DTE) to a modem/phone line (DCE) to a remote device such as a terminal (DTE).

In asynchronous, full-duplex communications, the most common method of flow

control between hardware components is by use of the Electronic Industries

Association’s EIA-232-D Request to Send (RTS) and Clear to Send (CTS) circuits.

Both hardware and software flow control (

<Ctrl>-<S> and <Ctrl>-<Q>, respectively) can be

used simultaneously.

Hardware flow control is implemented by RTS and CTS lines. Asynchronous ports

allow bidirectional hardware flow control. This bidirectional flow control means

that either the DCE or DTE can indicate to stop transmitting data across the

interface.

For more information, see the termiox(7) man page.

Null Modem Cabling

The asynchronous serial port is assumed to be configured as a DTE. If the

connected device is also a DTE, DTE-to-DTE hardware flow control is only

possible by using a null modem to interconnect the appropriate data and control

circuits. Connecting a terminal to the system is an example.

To connect a terminal to the asynchronous port, you must use a null modem cable.

If you are connecting a modem (DCE device), connect it directly to an arm of the

octopus cable.

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