Code is available on github.com/yottatsa/kr1801 and for direct download.

K1801VM1 to ECB considerations

K1801VM1 is a clone of PDP-11 CPU board that uses Q-bus, also known as LSI-11 Bus. It uses multiplexed A/D bus, and the bus is mastered by the processor card.

RetroBrew EuroCard Bus is more-or-less versatile 16-bit demultiplexed parallel bus with existing peripheals and reliable DIN 41612 connector.

K1801VM1 provides enough signals to easily latch and buffer multiplexed inverted bus to ECB. Control and interrupt signals are largely interchangeable. However, absence of separate IO channels poses a trick to interfacing with IO cards. Note that K1801VM1 always(1) expects devices to confirm the transaction, which might pose the need for special peripheals or additional glue.

K1801VM1 bootstrap

Few actions are needed to boot the processor after the power on or manual reset (pulling down RESET line):

1. RESET line is wired to DCLO signal and therefore it releases to inactive high
2. processor releases INIT signal, which is wired to RESOUT
3. together with releasing RESET, GAL asserts NMI to active low, this line is connected to ACLO and need to be deasserted to start the processor
4. operator indication NMI LED is turned on
5. pressing NMI button releases the line bus-wide and releases the processor