Tech Briefs

Alternative Controller for a Fiber-Optic Switch

This controller communicates via a serial instead of a parallel port.

NASA’s Jet Propulsion Laboratory, Pasadena, California

The figure is a simplified diagram of a relatively inexpensive controller for a DiCon VX (or equivalent) fiber-optic switch — an electromechanically actuated switch for optically connecting one or two input optical fibers to any of a number of output optical fibers. DiCon VX fiber-optic switches are used primarily in research and development in the telecommunication industry. This controller can control any such switch having up to 32 output channels.

altA digital input/output (I/O) interface circuit is needed for controlling the switch and reading its status. This controller was developed as an alternative to the control interface suggested by the manufacturer of the switch. The manufacturer provides a schematic diagram of an interface circuit that utilizes the parallel port of a personal computer, and provides sample software for use with the interface. However, the parallel ports on some personal computers are not compatible with the interface suggested by the manufacturer. In contrast, the present controller uses a standard RS-232 serial interface, which is available on most computers and can ordinarily be utilized with less difficulty than can a personal-computer parallel port.

The heart of this controller is a commercially available microcontroller that includes 16 digital I/O ports and flash memory that can hold up to 1KB of program. The controller also includes an RS-232 driver. Two of the digital I/O lines are used for serial communication, via the RS-232 driver, between the microcontroller and the personal computer. Nine of the digital I/O lines are used for controlling the switch and reading its status. One of the digital I/O lines is used to control the power to the switch through a relay, and two of these lines are used to drive a two-color light-emitting- diode (LED) front-panel display. The software enables a user to interactively control the switch by means of simple commands and to monitor the responses of the switch. The front-panel display enables the user to determine the status of the switch (“busy” or “error”) at a glance.

The serial-port architecture of this controller facilitates establishment of an interface between this controller and commercially available laboratory automation software. It is also possible to connect this controller to a universal serial port via an appropriate converter. Therefore, the switch can be controlled by a variety of computers, without need for expensive digital I/O boards or complicated programming.

This work was done by Robert Peters of Caltech for NASA’s Jet Propulsion Laboratory. NPO-43306