Sunday with my SuperBoard II

[stm]

The device operating the DD line must be open collector or open drain. The DD, when used with the 32K RAM board is pulled high by a resistor on the 600 board so you cannot use a device with an active pull-up.

So if I want to drive the 5V DD line with a Raspberry Pi 3.3V GPIO output, I would have to implement the open collector circuit with an extra transistor and pull-up resistor, is that correct?

[BillO]

There is a pull-up already on the 600 board, you should not need another. But yes, it would be easiest to take the GPIO of the RPi trough a 2n3904 transistor or small mosfet, like a 2n7000. If you wanted to get deeper into the RPi, there may be a way to turn off the active pull-up on it's GPIO lines. They may then also be 5V tolerant. I'm not sure about that but this capability exists in most microcontrollers like AVRs and PICs.