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RE: [N8VEM-S100:6167] 8080a CPU Board ROM 27256?

Hi Jeff,

Sure it will work.  Like you said, just a little more power hungry.  Speed? Not to worry, the CPU is just at 2Mhz.  I would be concerned if you were trying to use 1702's or perhaps 2708's with access speeds greater than 500nSec.  The "-2" usually means 200nS.  Without looking at the data sheet... at 2Mhz, 1 cycle is 500nS, about 1/3 of that is used to clock the Status Byte, the rest is for Memory Read, so I guess the maximum access speed that would work to be about 300nS.  Even playing it safe at 250, a 200nS device would work flawlessly.  Keep in mind, this parameter isn't like loading a power supply (ie, the closer to the limit, the bigger the heat & stress)... it's more like driving on the highway, if you are at or under the speed limit, you will not get a speeding ticket.  If you keep to the speed limit, you can drive to the four corners of our continent.


Date: Sat, 31 Jan 2015 18:05:44 -0800
From: jeffrey.h...@gmail.com
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:6167] 8080a CPU Board ROM 27256?


Digging around the shop I found my old EPROM eraser, B&C Micro 1409 programmer and a bunch of 2764 and 27256 (D27256-2) proms. Tested the programmer, got a menu and found documentation on the web.

In the FIRMWARE and "CPM V-Disk Images AIR-V23.zip" archive in the ReadMe.txt I found reference to 25c256. Can I perhaps use the 27256 I found? It's about 1/2 the speed and more power hungry. Pinout looks the same.

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