RS-232 Data Switch

  
This report deals with the design for a small asynchronous serial data switch. The switch architecture allows 14 serial ports supporting a subset of the RS-232 protocol. The requirements for the switch are discussed. The hardware interfaces are described in detail, and the software function is described in the form of a user's manual. Finally, the hardware and software implementation is presented. It is concluded that though low-cost, the hardware design is excessively complex. It is found that the simple 3-wire serial interface is sufficient for most applications, and that an RS-232 `break` signal is ideal for obtaining the switch processor's attention. It is seen that a clock/calendar capability could be added at zero cost to the switch design. It is recommended that an improved switching network design be investigated, that the software functionality be enhanced to allow better security, and that CMOS integrated circuits be used to reduce heat dissipation.
RS-232 Data Switch - schematic

It was found that a 3-wire serial interface would be sufficient for each of the data ports. Two additional status lines were implemented mainly for the use of the switch. The remaining status lines were tied to the appropriate logic levels with resistors. It was found to be trivial to make the ports immune to damage due to wiring errors. It was decided that requesting the switch's attention should be done via the serial data line. The only serial activity which the switch can monitor on all lines simultaneously is a break signal. This was used. The length of the break signal required was made configurable, so if it is possible to produce variable length break signals, short ones can be sent without dropping into command mode. A considerable amount of hardware was needed to build a six port version of the switch. Thirty-four IC packages were needed. Implementing the remaining eight ports would require another twenty-six IC packages. Due to the requirements for the switch processor, it was possible to implement a real time clock/calendar at zero hardware cost. Since the switch will be powered up at all times, this will be a useful feature. RECOMMENDATIONS It is recommended that the switch software be expanded to provide complete security provisions, so it can be used in a less secure environment than that originally aimed for. Most of the required functionality is already in the software. A more elegant solution...



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