Gregory's ServerUPS_CD_SEL# from the point of view of the downstream sidecar means that the sidecar upstream has decoded the current address. if this signal is active and your own address decoder is also active, that means that both expansions are trying to decode the same address, which is a conflict! one of the PALs detects this and asserts BERR# (bus error).
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 i've been reading the Amiga 1000 Assembly and Component Level Repair manual and found this interesting memory map diagram. notice what is stored from F00000-F7FFFF? it is marked "CART." could this have been for a game cartridge slot? the Comspec SCSI card puts its autostart ROM here. @tubetime most likely it is a remnant of the old game console design, but it was also heavily used in os development for temporarily loading new ROM modules when testing, and also many expansions used it for their roms if they did not do autoconfigure. here's the Comspec SCSI drive enclosure. it's a bit of a beast and supports two half height drives. Comspec was definitely Canadian; check out that mounting screw!  @tubetime Brings back many fond memories of ST-225Ns! And the vias on that board are strangely attractive… the circuit is similar to the "typical application circuit" and it looks like there are two small electrolytic capacitors on the UC3842 VCC rail that have gone super leaky, preventing it from bootstrapping and powering up. it was a simple enough fix. the yellow screen means it is trying to load the Kickstart off of a special drive partition. oh that's not good, it blinks red 8 times. this means the drive failed the SCSI mode sense command for some reason. weird. @tubetime Is that something special added by the controller interface? Or is the onboard boot room able to look for kickstart other than the floppy disk? @ChartreuseK the onboard (non kickstart) ROM looks at F00000 and finds the controller ROM, which then takes over the boot process. i think this was originally for supporting game cartridges @tubetime Ah okay, didn't know that was a thing it did. Certainly a nice feature to not need the kickstart disk to boot a hard disk  @tubetime I've run into this in audio amplifiers too. They have a bootstrap pin which is connected to a RC network off a supply voltage - the cap charges up slowly once the power comes on and eventually lets the amp come out of inhibit. It's a power up thump avoidance thingy. And yeah, when the cap fails short, the amp plays dead |
here's how the address gets decoded. the register on the left is normally disabled, so the pullup and pulldown resistor arrays set up the address comparator to "E8" which is the autoconfig space. during configuration, the card gets assigned a new address which gets latched in the register and overrides the default address. simple and elegant.