In the on-board circuitry, the motor controllers receive their commands from the decoder (see Figure /reffig1(b)). The control bits at the LMD 18245 input pins are supplied by the decoding circuitry (Figure 6) only after the decoder compares the serial input information twice with its local address and no error or unmatched codes are encountered.
Figure: The decoding circuitry.
Simultaneously, a valid transmission indicator will go high allowing a LED to be activated on the agent's board. The latch type data outputs follow the coder during a valid transmission and are latched in this state until the next valid transmission. All the decoder circuitry (buffers, comparators, oscillator, data detector, data shift register, latch circuit, transmission gate circuit, control logic, divider and synchronization detector) is implemented by a HT648L (Holtek) CMOS LSI integrated circuit. The low power, high noise immunity CMOS technology along with a low stand-by current and an operating voltage range of 2.4 to 12 V makes it a very good choice for remote control systems. Its built-in oscillator needs only a 5% resistor to set up the desired frequency. In our design, a 100 kHz oscillator frequency is selected by a 330 kOhm resistor. The oscillator is disabled in the stand-by state and activated as long as a logic high signal is applied to the data input. Even though the decoder is capable of validating up to 10 bit addresses, only 4 bits are used to address up to 16 robots. The extra bits enable future expandability of the system. The decoder's activity flowchart is shown in Figure 7.
Figure: The decoder activity flowchart.