Thanks. So the hardware reset from the rest of the BBC Micro (8271 socket pin 4), which appears to be active high, at least on that pin, is inverted and fed to the 1170 MR pin which is active low. So a power-on or the break key will reset the 1770.8271 pin 4 (reset) is connected to pin 5 of a 74LS04 inverter. Pin 6 of this inverter is connected to 1770 pin 13 (master reset).
What I think we believed was that 1770 pin 13 (MR) was also driven from a latch (D-type, 74LS173). Other outputs from this latch control the drive and side select lines to the 8271 socket, and the pin to select MFM recording (DD) on the 1770. But if pin 13 of the 1770 is connected to the output of a simple inverter as you describe, it seems unlikely the D-type would also be connected to it in competition.
I am beginning to think this impression comes from a belief that this board was "just like the acorn one but with the D-type bits in a different order".
Statistics: Posted by Coeus — Wed May 29, 2024 11:59 pm