rom switching tutorial please

[selfpreservation]

hi can anyone explain in very simple terms  how to build a rom switching setup, i want to add extra sounds to my RZ1 i understand the rom ripping and burning side( i think )but i cant understand how to switch between the roms ?, is it just as simple as switching power on and off to the roms ie route the power to just one pin on one chip or the other possible via a rotary switch ? or do you have to switch all the pins in each rom in/out which case i havent a clue 

cheers ciaran

[Circuitbenders]

read up about 'chip select' pins

http://en.wikipedia.org/wiki/Chip_select

[Gordonjcp]

A lot of the ROM chips used in drum machines follow the standard JEDEC pin layout.  The upshot of this is that if you have a 256kbit ROM in your drum machine, you can swap it for a 512kbit EPROM and you'll have an extra address pin left over.  If you leave this pin disconnected (make sure the pin on the socket isn't connected to anything) then it will float high, and select the upper half of the addresses.  A pullup resistor to +5V (1k should be fine) will ensure it is pulled high, and you can then add a switch to pull the pin to ground to select the lower half.

[selfpreservation]

ah ha  very interesting is the chip select way the only way to get more than 2 sound sets or can one get a rom with 3 or more 256k address lines and switch between them ?

[Gordonjcp]

Yes, obviously if you've got two "extra" address pins you can switch between four banks using the same technique.

It's worth bearing in mind, too, that although the pin numbering goes astray a bit you can often fit a 32-pin JEDEC EPROM into a 28-pin socket with the pin 1 end overhanging the end.  You need to make some other arrangements for connecting power etc. but if you follow the pinout diagrams you won't go far wrong ;-)