On Tue, May 19, 2015 at 11:28 PM, Mike Stein <mhs.st...@gmail.com> wrote:
> My 8041A/8641A/8741A data sheet says
> "interchangeable ROM and EPROM Versions", "Single
> 5V Supply" and "Vdd (Power): 5V during normal
> operation".
"Normal operation" is the key phrase. ROM (or EPROM) verification mode
is not considered "normal operation". In normal operation, the
8041A/8641A/8741A is executing the ROM/EPROM code, and you can apply
an address externally to the port pins until the cows come home, but
that won't make it dump the contents of that address.
> I'm pretty sure I've read 8741s without any 25V
> supply; AFAIK Verify mode is not the same as Read ('normal') mode.
This isn't a 2716. There isn't a "read (normal)" mode. Normal mode is
executing code, and doesn't allow reading the ROM (or EPROM) contents
externally. Verification mode is the only mode that provides for
external access to the ROM/EPROM contents. The only documented way the
chip knows to be in verification mode is a supervoltage on the EA pin.
A National Semiconductor app note suggests that 9V is adequate for
reading their 804x masked ROM parts; they specifically mention
internal circuitry that detects a voltage on EA above a certain
threshold which puts the part into verification mode. The 9V National
describes might or might not be adequate to read Intel masked ROM
parts. Intel specified 12V to read the 8048. Intel neglected to
specify the supervoltage required for ROM verification mode for the
8041 and 8041A, but I'm pretty sure they expected 12V for those as
well.
