Re: II+ vs IIe Char Gen PROMs and Shift-key mods-- LC $F8 PROM mod

[Rubywand <rubywand@swbell.net>]

David Wilson writes ...
> 
> Rubywand <rubywand@swbell.net> writes:
> >David Wilson writes ...
> >>The only problem with this change is that the ROMs in a ][ or ][+ are not
> >>pin-compatible with EPROMs of the same size. Thus, to make this change you
> >>would need to wire an adapter between the EPROM and the ROM socket to sort out
> >>the chip select polarities.
> 
> > True. The 2516 and 2716 2k x 8 PROMs come close.
> 
> > One problem is that the ROMs have three enable/select pins; the PROM has two.
> 
> > Fortunately, the Apple II uses only two select lines; so, the main thing you
> >need to add to an adaptor is an inverter-- like a 74LS04 or any other inverting
> >gate.
> 
> Acording to Jim Sather, the inverter can be dispensed with:
> 
> Pin #   ROM socket              2716 (except TMS2716 - use TMS2516 instead)

     Definitely. The PROMs to use are the ones which match the TI 2516 / Intel 2716
pinouts. For some weird reason, TI's 2716 uses a different pinout.




> 18      CS2 (active high)       /CE (active low)
> 20      CS1' (active low)       /G (active low)
> 21      CS3' (active low)       Vpp (25v prog, 5v read)
> 
> So if pin 18 (ROM) goes to pin 21 (EPROM) and pin 20 (ROM) goes to both
> pin 20 and 18 (EPROM) it should work.
> 
> Looking at the 2716 data sheets I have this is not valid as Vpp is only
> defined at 5v and 25v, not at 0v. If you do not have any cards which drive
> the INHIBIT' line then this will not be a problem.
>

     Jim's idea of using the programming voltage pin as a Hi chip select might work--
have not tried it.

     As you say, the manuals insist that it is supposed to be held at +5V during
normal operation. If the line were a reliable Hi select, you would think this would
be mentioned as a major feature, since it would make the PROMs directly swap-able for
many ROMs.

     My guess is that, at least on some of the PROMs, pulling the line low turns Off
all drive to the outputs. The required levels for Hi and Lo to use this pin as a
Select do not seem to be documented-- so, for example, we do not know whether a
glitch to, say, 2.7V will be taken as a Lo and suddenly flip Off the outputs. How
fast turn-Off and turn-On happen using this pin as a select is not documented either.

     If Jim says pin 21 works for such-n-such a PROM as a Hi Select, it is probably
okay to use it; but, I'd rather just tie it Hi (+5V) and use the documented Selects
for chip and chip output even if, sometimes, it means adding an inverter IC.

 
> >Probably, a II+ will have the Language Card plugged into Slot 0. With the LC
> >plugged in the $F8 ROM used is going to be the one on the LC; so, this is
> >where you would swap in the adaptor with the PROM.
> 
> If you have a Language Card, it already has two solder points to convert it
> to 2716 polarity. A third point marked F8 should be left open unless you want
> the motherboard F8 PROM used when the motherboard BASIC is selected.
 ....

     Neat to know! Is this stuff documented somewhere on-line?



Rubywand