My Tindie shop is out of stock, so I decided to fabricate a new batch of PCBs. I have skipped the Issue 3 (backplane version) and made a new, slightly improved version, Issue 4. The improvements are:

• Signals RESET and READY are added to the application / system bus
• 10k pull-up resistor between READY and Vcc to drive this line with open collector drivers
• Removed the LED and 330R resistor. Three pins are added: SID, SOD and GND to enjoy a serial port.
• One place for 22pF capacitor was removed.