Revealing the HP-82242A IR Printer Module

[brouhaha]

(05-24-2024 09:38 AM)ThomasF Wrote:  It should be noted that only a selected peripheral should actually drive the FI bus signal (apart from FI 13), i.e. if both Time and Blinky modules are present, it is only the currently selected module that should drive FI when needed (this is to make sure that two modules shouldn't compete about the bus signal).

Selected how? What specific condition would the module use to decide whether to drive FI? It could be that the IR module works that way, but I somewhat doubt that, as the other modules do not.

The Nut CPU precharges the FI line to the inactive state, and devices only drive to the active state, never the inactive state, so there is no electrical conflict if two devices share the same flag, with only one driving it active, and the other not. This is similar to "open collector" TTL, or "open drain" CMOS, but of the opposite polarity.

The same precharge is also done on ISA and DATA, though on those, the selected device drives both low and high. The precharge on those is used to guarantee that reading nonexistent ROM or registers reads all zeroes; without precharge the result could be unpredictable. The Nut uses instruction 000 as NOP, except that if a JSB goes to a 000, it is instead treated as a return. This is used so that the mainframe can call the service ROM, and ROM polling entries, even when the ROMs are not present (or a ROM poll entry uis empty)

In the Classic and Woodstock generation PMOS chips, even the ROM and register chips only drove outputs active, and never inactive, in the same way as the Nut FI. In PMOS, driving high with relatively normal-sized FETs works well, but relatively huge FETs are needed to drive low. They avoided needing the large FETs in the ROM/RAM/peripheral FETs by having only thr CPU drive those low. In CMOS (including the Nut), the transistor sizes needed are comparable for both high and low, so they didn't omit any, but for the deliberate choice on the FI line.

The time module alarm flag is documented in HP's timer chip ERS, which was leaked and is available from the usual archive sites. I have not studied the time module code to check for actual flag tests, but the VASM listing is available.

Thanks for the continued work on understanding Blinky, and sharing your findings!