The most basic mode of operation for a BrainStem module is as a slave device that responds to commands.  All modules continually check for incoming commands.  When a module receives a command, it responds by performing a simple I/O or system task.  Such tasks can include moving a servo, returning an A2D reading, or launching a TEA program.

Commands are sent as packets of binary bytes.  They can be sent from the host via the router, or from another module on the bus.  They can also be generated internally with reflexes or TEA programs.