CNROM PCB without a '161 still good for NROM use?

nintendo2600 · Post by **nintendo2600** » Fri Apr 09, 2010 1:42 am

Hey All,

So I have a CNROM pcb but the '161 was removed long ago. What I was wondering is this CNROM without the '161 still good to use as an NROM board or is it just worthless at this point? Just wanted to check before I throw the pcb in the trash.

Thanks!

Post by **tepples** » Fri Apr 09, 2010 5:40 am

You might have to rewire the CHR address lines so that Q1 and Q0 out of the 161 are in a known state (for chip enables), but yes, it should still work.

coinheaven · Post by **coinheaven** » Fri Apr 09, 2010 7:01 am

so normal cnrom boards can be used for nrom games?

MottZilla · Post by **MottZilla** » Fri Apr 09, 2010 11:21 am

Sure they can. If you just duplicate the 8K CHR to fill up the chip or only have a 8Kbyte chip it should function the same.

nintendo2600 · Post by **nintendo2600** » Sat Apr 10, 2010 10:42 pm

tepples wrote:You might have to rewire the CHR address lines so that Q1 and Q0 out of the 161 are in a known state (for chip enables), but yes, it should still work.

Can you please expand on this?

Post by **tepples** » Sun Apr 11, 2010 5:42 am

Your 8 KiB EPROM has 13 address pins, 8 data pins, and whatever other pins are needed for power, ground, and enable signals. The ROM that came in your board has two additional address inputs A14 and A13; these probably correspond to extra enable signals on the EPROM if anything. But now that you've desoldered the 74HC161, these signals are floating. In general, all input pins should be connected to something to ensure predictable operation. You'll need to connect these pins to either power or ground to make sure the enable signal is what the EPROM expects.

nintendo2600 · Post by **nintendo2600** » Tue Apr 13, 2010 7:48 pm

tepples wrote:Your 8 KiB EPROM has 13 address pins, 8 data pins, and whatever other pins are needed for power, ground, and enable signals. The ROM that came in your board has two additional address inputs A14 and A13; these probably correspond to extra enable signals on the EPROM if anything. But now that you've desoldered the 74HC161, these signals are floating. In general, all input pins should be connected to something to ensure predictable operation. You'll need to connect these pins to either power or ground to make sure the enable signal is what the EPROM expects.

Ah ok, gotcha. I'm gonna tie them to 5v then. Thanks for explaining.