Sierra Wireless DART 300 Serial Connector, Null-Modem Connection, User’s Guide Device Attachment

User’s Guide Device Attachment

10.3. The Serial Connector

When testing the modem using a PC terminal, the cable is a standard RS-232 9-pin connection as outlined below:

Pin	Signal	Direction
1	DCD – Data Carrier Detect	DCE ! DTE
2	RxD – Receive Data	DCE ! DTE
3	TxD – Transmit Data	DCE " DTE
4	DTR – Data Terminal Ready	DCE " DTE
5	GND – Signal Ground	Common
6	DSR – Data Set Ready	DCE ! DTE
7	RTS – Request to Send	DCE " DTE
8	CTS – Clear to Send	DCE ! DTE
9	RI – Ring Indicator	DCE ! DTE

The pins are arranged in the connector as noted below:

5 4 3 2 1

9 8 7 6

Figure 10-1: Pinouts for Female DB-9 Connector (DCE).

10.3.1.Null-Modem Connection

When connecting the DART 300 to a local host device, there may be a need to use a “null- modem” type of connection. The RS-232 interface specification was intended to standardize the method that computers and terminals (DTE) used to communicate remotely over telephone lines (DCE). The popularization of RS-232 serial communications resulted in many different types of equipment, besides modems, that employ the DCE standard. Many manufacturers expect devices to connect directly to DTE equipment (usually, PCs). Consequently, devices are designed with a DCE interface, so a standard RS-232 cable meets the cabling requirements.

If the local device is designed for connection to a DTE terminal, it is already designed to be a DCE device. Since the DART 300 is also a DCE device, the connections must be crossed over to enable throughput. To identify this problem, connect a PC directly to the local host device and establish communication. If communication is established, the device uses a DCE interface and the null-modem cable will be required to connect to the DART 300 modem.

Null modem adapters are available that look like a RS-232 gender-changer. These adapters are available from electronics stores and are well suited for application development. Null modem cables can be ordered from cable suppliers and electronic stores. These are better suited for field deployment because the cables eliminate the extra part and potential for failure that an adapter would introduce.

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