CDPD Modems And Remote Ethernet Devices

Abstract: CDPD modems are often used to provide low speed connectivity to serial Remote Terminal Units (RTUs) and other control and data acquisition devices. Most CDPD modems are designed to connect to remote devices (such as a laptop)using ppp, and with the proliferation of ethernet-enabled remote devices, a method is needed to connect the serial RS-232 CDPD modem to one or more ethernet-enabled devices at remote sites along with serial devices. The DCB IP-5200 micro-router application described in this paper is an example of this technology in use. Along with the IP-5200, an EtherPath ethernet to serial converter may be used for direct access to serial devices.


An oilfield injection well management system requires bi-directional data transfers on a daily basis. This data is analyzed in real time and used to tune the injection process for optimum production. CDPD was the most cost-effective, reliable communications network available in this area and was selected by the customer due to cost and coverage reasons. The CDPD modem used in this application was the RAVEN by Airlink.

The remotely located PLC used in this management system was manufactured by Siemens and instead of an RS-232 port, contained an ethernet port. This versatile PLC was programmed for automated data pull operations by the management system, and the customer wanted to use its self-contained web server for ad-hock data inquiries.

The selected solution used the DCB asynchronous router between the CDPD modem and the ethernet port on the PLC. This device routes the PLC data on ethernet to the CDPD modem, changing the IP addresses as needed and also providing "firewall" protection. Since the CDPD modem is configured for a serial PPP connection, the IP-5200 automatically connects as needed to the CDPD system and transfers the requested data.

The CDPD system is connected to the Internet, so field data is available (as the security policy defined in the IP-5200 allows) to virtually anyone via the Internet. One consideration to be noted is the relatively low bandwidth of CDPD networks. Although the IP-5200 can handle data at 230 Kbps, the CDPD network limits the rates to 19.2 Kbps or less.

A side benefit provided by the IP-5200 is Network Address Translation, or NAT. This enables multiple ethernet devices to be used behind a single CDPD modem (bandwidth allowing).

This is another example of a resourceful design engineer solved a common problem (remote site communications to new equipment) with an innovative application of standard products.

Data sheet for IP-5200 router. Data sheet for Etherpath serial server.

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