Electronic Design

Optical-Fiber Link Switches Multiple RS-232 Ports

A recent application required a switch that would allow multiple RS-232 serial data links to share a single physical link. This switch was necessary to reduce the cabling between the instruments on the focal plane of our upcoming 2-m optical telescope and the computers in the control room. One possible solution involved using a high-speed microcontroller to implement priority encoding of the incoming serial data. A corresponding decoder was placed at the other end. The encoder and the decoder were connected by a high-speed serial or parallel link.

Of the design options available, we decided to use our optical-fiber data link experience to build this switch. The highspeed optical-fiber link is based on the AMD TAXI transmitter and receiver (see the figure). Also available from Cypress are compatible and upgraded components such as the Hotlink chip set. When an external strobe signal is activated, the TAXI transmitter encodes the data input bits using a 4B/5B encoding scheme.

This encoded data is serialized and transmitted to the receiver through an optical transceiver at 10 times the byte rate. Therefore, a byte rate of 10 Mbytes/s results in a bit rate of 100 Mbits/s. Once the receiver obtains the incoming serial data, it performs a serial-to-parallel conversion and decodes the data. It then outputs the data on the output lines together with a strobe signal.

The switch can be implemented in a variety of ways. In this simple application, for example, the data sent to the instruments from the PC is level-shifted using an ADM232 TTL-to-RS-232 converter (see the figure, again).

As many as eight such signals can be connected to the data input pins of the TAXI transmitter. Every 200 ns, a locally generated strobe signal activates the transmitter. The data is latched into the transmitter, which encodes, serializes, and transmits the data. Then, the TAXI receiver on the instrument end receives the data and latches it into an external register (74HCT373). This drives another set of ADM232 level-shifters to regenerate the RS-232 signals and distribute them to their respective instruments.

Similar data translation occurs for data flow in the opposite direction, from the instruments to the PC. The flow control between the RS-232 transmitter and the receiver must be implemented in software (Xon/Xoff flow control).

A 100-Mbit/s TAXI-link switch with up to eight RS-232 inputs and outputs is used for this circuit. The system introduces a maximum of 200-ns jitter in the RS-232 timing. This safely allows simultaneous RS-232 speeds of up to 100 kbits/s on all eight RS-232 channels. Another feature of this switch is the inherent electrical isolation between the instruments and the control computers.

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