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Wiring a 27C512 to get 2 banks

S

Sharkonwheels

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OK - can anyone assist?
Want to write (2) diff programs to a 27C512 for my NEC 8201a, and
wire up a switch between the (2) 32KB sections.

Seen this:

http://symlink.dk/nostalgia/c64/rom/

scroll to middle, and this (which above is based on):
http://www.64hdd.com/images/kern-3x8.gif

ignore the center 24-pin section.

Now, it looks wiers, because they show /CE going to ground, and then through a switch and a resistor, hitting Vcc.

This all look right?
If I wire this up, do I go to A15 and A14 for the switch, and leave them out of the NEC's socket? Also /CE?

T
 
Now, it looks wiers, because they show /CE going to ground, and then through a switch and a resistor, hitting Vcc.
This all look right?
Click to expand...
That's right, for a 27C512/2364 adapter.

The common pin of the switch is grounded. The two other wires from the switch (and the resistors) goto the A13 and A14 pins of the 27256 socket and no where else.

And /CE of the 27256 socket is tied to ground. Why? If you look at a 27256 data sheet, the 27256's mode of operation is controlled by three pins: CE (pin 20), OE (pin 22) and VPP (pin 1). You want READ mode. For READ mode, VPP is set to the normal voltage, /CE is taken low (ground in this case), and then /OE is taken low whenever the 27256 is to put data onto the data pins.
So in READ mode, the 27256's /OE pin is equivalent to the 2364's /CE pin.

Want to write (2) diff programs to a 27C512 for my NEC 8201a, and
wire up a switch between the (2) 32KB sections.
If I wire this up, do I go to A15 and A14 for the switch, and leave them out of the NEC's socket? Also /CE?
Click to expand...

In the Internet sites you quoted, 'bank switching' of a 27256 (32K) to a 2364 (8K) is occurring.
You want to 'bank switch' a 27512 (64K) to a ???? (32K)
What is the ???? chip? Some of us are unaware of the NEC 8201a, and so we can't presume that the ???? chip is a 27256.
In any case, it's just the A15 pin you will be controlling - to select which of the 32K portions of the ???? chip you want.
But we need to know what the ???? chip is to determine exactly how the adapter will be wired.
 
I was just posting a reply to this, but then thought "what if the 8201 uses *CE?" (the kludge is for a commodore 64) so I was going to ask if anyone had a link to the 8201 schematic?
 
Heh - apologies.
8201a uses 27256's

I was only referring to the 28-pin portion of the drawing.

docs are at www.web8201.com at http://web8201.com:8080/Files/public/nec8201a_service_manual_with_schematics.zip

got the 27c512 working...
Put an SPDT switch, center to A15 (which is out of the socket), one side to Vcc, one side to GND.

Now, if i could kludge up a 27C010 for (4) rom images....
A bit more complicated, I guess, as now there's more than 1 to drop Vcc and Vss to!
I assume I'll need (2) SPDT switches for this one. Now, A16,A15,A14 will all be out of socket, correct?
Looks like I will also have to bring the +5v to pin 32 (Vcc) and pin 28 is normally Vcc on a 27C256/27C512, but N/C on a 27C010

Never happy, are we?


T
 
Last edited:
Sharkonwheels said:
Heh - apologies.
8201a uses 27256's

I was only referring to the 28-pin portion of the drawing.

docs are at www.web8201.com at http://web8201.com:8080/Files/public/nec8201a_service_manual_with_schematics.zip

got the 27c512 working...
Put an SPDT switch, center to A15 (which is out of the socket), one side to Vcc, one side to GND.

Now, if i could kludge up a 27C010 for (4) rom images....
A bit more complicated, I guess, as now there's more than 1 to drop Vcc and Vss to!
I assume I'll need (2) SPDT switches for this one. Now, A16,A15,A14 will all be out of socket, correct?
Looks like I will also have to bring the +5v to pin 32 (Vcc) and pin 28 is normally Vcc on a 27C256/27C512, but N/C on a 27C010

Never happy, are we?

T
Click to expand...
---------
Yeah, pin 1 is your friend, although you don't need a DT switch; a ~10K resistor to Vcc and a SPST switch to ground will be OK.

I've done this with several systems and have been meaning to do it on my M100s (and fix the Y2K at the same time) but it's a little more work because of the non-std ROM.

For the 1Mb EPROM I'd just piggyback a couple of 512s (if you've got room) and bend out pins 1 & 20; connect them to a pair of DPDT switches and ya should have 4 ROMs.

mike
 
Last edited:
Actually, I got it working with an St M27C1001 1MBit EPROM, and got all 4 banks!

This shiznit is COOOOOOOL!

On the 8201a, I wanted to make it look as factory as possible, so I drilled 2 holes, on either side of the DC jack, and used (2) SPDT mini toggles.
Center on one goes to A15, center on other goes to A16.
other poles go 1 to Vcc, one to Vss.

Shiznit works GREAT!
Turn it off, flip the switches, and VOILA! Another image!

Currently I have:
TS-DOS
Ultimate ROM 2
ROM2/Cluseau
Sardine

Here's a pic of how the switches came out:
IMG_1063.jpg


Pictures at:
http://s92.photobucket.com/albums/l33/Sharkonwheels/NEC PC-8201a 4-Bank EPROM/

T
 
Hi
The diagram looks right to me. The 2 address lines are selecting
one address line to be high.
I think I might make it so that one can get 4 rom options instead
of just 3 as the diagram shows.
The connection of /CE goes to ground and is not effected by
the switches. The switches are for segment selection of the address
lines. The switch is also connected to ground. It isn't that the /CE
goes through the switches, it is just connected to common ground.
Dwight
 
Thank you, thank you <taking a bow>

Got help on the wiring from Stephen Adolph, guru of all things M100...

Next, I;m gonna try a 29C010 or similar flash chip, and see how that works.
Same pinout, easier to reprogram

T
 
Now I'm wondering if something similar can be done with a PX-8.
Is it OK to flip the switch while the system is powered on?
Now, on a PX-8, flipping the switch and doing a <Ctrl><C>, ya think it
will recognize that the ROM changed? That would be a cool way to have all the ROMs available at once.

Anyone got a PX-8 they wanna donate? Doesn't need to be pretty - actually, I'd prefer it NOT be ;)

T
 
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