While most of our HCS-II products are no longer manufactured, we still have certain upgrade kits available for existing HCS-II users. The PL-Link modification kit includes the MAX813L chip, 8-pin IC socket, and 10K resistor and can be purchased for $5
A few people have had stability problems with the (now discontinued) PL-Link X-10 Interface for the HCS-II. Often, the PL-Link would ‘freeze’ in transmit mode with the TW-523 constantly flashing and the X10 Transmit status line pegged high. After lots of debugging by various HCS-II users, it was determined that the PL-Link was very susceptible to power noise and glitches. The PL-Link reset circuit is a simple RC setup which doesn’t behave well during a power fluctuation and will often ‘half-reset’ the CPU meaning it never issues a complete reset pulse which locks up the board.
The best solution put forth so far is to replace the reset circuit with a microprocessor supervisor chip. I’ve chosen the MAX813L from Maxim. Adding this chip to your PL-Link will ensure that a solid RESET is generated if the supply voltage dips below 4.65V. The MAX813 also has a watchdog circuit. Should the PL-Link ever lock up, the MAX813 will RESET the PL-Link with a good clean RESET pulse. Of course you should also try to clean up your power supply if possible. Some folks have gone as far as using Gel-Cels though my system is just fine with a switching supply.
This is a VERY easy modification. If you have a PL-Link, I highly recommend it. My PL-Link has been rock solid since I made this modification. The schematic diagram includes detailed instructions for the modification.
