I checked the Commodore 128 schematics last evening as they were made with an option ROM socket inside which could accept EPROMs. It was also not uncommon for them to have EPROMs fitted for the main system ROMs form the factory when they were low on masked ROMs.
They have VPP tied to +5V on all ROM sockets stating it is for VPP even though they show that pin as NC for the ROMs. I would take that as evidence pulling it up to VCC is a good idea. As I recall on the T102 the VPP pin is not connected to anything, but a little bodge wire will fix that and not interfere with an original masked ROM either. Jeff Birt From: M100 <[email protected]> On Behalf Of Stephen Adolph Sent: Thursday, March 10, 2022 10:01 PM To: [email protected] Subject: Re: [M100] 27C256 pin 1 VPP - can it float? I think that means the old 27C256 in the T102 is a bad idea, without an adapter board or a wire to +5V. On Thu, Mar 10, 2022 at 9:52 PM Brian White <[email protected] <mailto:[email protected]> > wrote: I always tie it to vcc, with a pullup so that a programmer can still override it. As you saw yourself, every datasheet always says to pin to vcc or lower, usually preferring vcc. I think the harm is just the risk of reaching vpp randomly if left flapping, so you might corrupt the data. Probably *almost* never happens, but the datasheet directions implies that there is no equivalent of a built-in pullup, which means the pin is free to spike even from say, static. Seperately, I would always nail down *any* input just on principle, and especially any control input. -- bkw
