Embedded computers and PDAs and have become popular as "brains" for robotics projects.  They usually have an RS-232 serial port that interfaces easily to a slave IO controller.  Unfortunately a single serial port can be a severe limitation if a user wishes to connect multiple serial devices.  This page describes one method to switch serial signals between a host device, such as a PDA or PC, and a slave IO controller or one other auxillary device with a serial interface. 

The switching circuit uses two of the multiplexers on a 74HC157 quad 2-to-1 multiplexer (or mux) chip.  One mux switches the host's serial reception input (HRX) between the slave device's serial transmission output (STX) or the auxillary device's serial transmission output (AUX-TX).  The other mux switches the auxillary device's serial reception input (AUX-RX) between the host's serial transmission output (HTX) and +5V.  On most devices with a TTL-level serial port, a signal of +5V is perceived as an idle state. 

The slave device must have one free digital output that can be switched on or off via a serial command.  This digital output must be connected to the Select line to control the multiplexers.  A low logic signal on the select line will enable communication between the host and slave.  A high logic on the select line will enable communication between the host and auxillary device.  The slave device will still receive data from the host when the select line is logic high.  This makes it possible for the slave device to detect a special sequence of characters from the host and disable communication with the auxillary device.  The auxillary device must be able to ignore this "escape sequence".  The multiplexer circuit is shown below:

This circuit was developed to route either BrainStem GP 1.0 or CMUcam serial signals to a PDA.  Some PDAs may require an RS-232 voltage level shifter for proper operation.  (The connections shown in the circuit for the slave and host are compatible with a BrainStem module and serial interface adapter.) It is left to the user to determine appropriate escape sequence logic for auxillary serial devices.