[Simh] DEC VT emulators on MAME

[Johnny Billquist]

On 2017-04-18 16:42, Timothe Litt wrote:
> You can remove the ROMs (EPROMS) nondestructively with hot air.  But an
> easier approach would be to tack some wires onto the connector & wire
> them to a 28 pin DIP header.  Then your existing programmer can read them.

That would be a pretty straight forward and safe solution.

> The schematics don't seem to contain the ROM module, but one can infer a
> lot from the connector.
>
> It looks like there are 2 banks of ROMs on the cartridge; there are also
> 2 select lines.  Address bits go to A15; the 27c256 uses a0-a14.

Didn't really look at the schematics for the connector yet, but a short 
question. Are we talking about A0-A15 going on the connector, and 2 
select lines in addition to that?
If so, then we have in essence 3 select lines, since A15 is outside of 
the addressing range itself.

So that would mean you probably have 32K always mapped in the low 
address space, and four different banks for the high 32K.

(But I'm just guessing here. Like I said, I haven't actually looked at 
any schematics or connectors...)

> So there are probably 2 chips on one bank, 3 on the other.  Since they
> don't write the EPROMs (they'e windowless, so OTP), they probably use
> A15 for OE and the bank lines for CS.

Since they're windowless, they are not EPROM (remember what the E stands 
for), but plain ROMs.

While it's possible they'd use A15 for OE, I would not think so. I would 
definitely pull A15 to CS for one of the chips, and always have the low 
32K mapped through that. And then the high 32K would be done through the 
two select lines. But that's just me guessing.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol