On Mar 4, 2:18 am, yuhong <[email protected]> wrote: > On Mar 3, 12:08 am, yuhong <[email protected]> wrote: > > > If you are wondering why did the Mac II(x) require PAL SIMMs for 4MB > > and higher, answers are in the end of this > > technote:http://web.archive.org/web/20040803140105/http://developer.apple.com/... > > You will see that it is more correctly 4 Mbits and higher SIMMs, as > > well as other details. > > BTW, I wonder if anyone has managed to hack a PAL into an ordinary > SIMM using the info provided here? > The hard part is going to be probably programming it.
To hack a PAL or PLD into an ordinary SIMM you'd have to cut the WE_ lines and reroute them through the PLD and at least tap into the CAS_ and RAS_ lines although there'd probably be no need to cut them. If you're able to cut WE_ right at the SIMM pin that's not too bad. There's only one (pin 21) at the SIMM, although it must fan out to eight, to supply each chip on the SIMM. Then I think you'd need a little state machine in the PLD. Something like, WE_OUT is high unless WE_IN is low AND CAS_ was low more recently (or currently) than RAS_. Depending on the timing of the memory operations you might be able to do this with a few SOT-23 logic gates. Let's see...WE_ can't be low when both RAS_ and CAS_ are low. And WE_ should only be low when CAS_ is low, although this latter condition may be wrong. I'd need to look at the memory datasheet, but I think CAS_ is held low along with WE_ while the column address is supplied, right? Wrong? So: RAS CAS WE | WE_OUT 0 0 0 | 1 0 0 1 | 1 0 1 0 | 1 0 1 1 | 1 1 0 0 | 0 1 0 1 | 1 1 1 0 | 1 1 1 1 | 1 And WE_OUT = RAS' OR CAS OR WE_IN So if WE_ doesn't need to be held low longer than CAS_ it looks like one could do it with an inverter and two OR gates or an inverter and one three input OR gate. Jeff Walther -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
