[Simh] DEC VT emulators on MAME

[Johnny Billquist]

Ok, looked at the schematics now.

On 2017-04-18 21:53, Timothe Litt wrote:
>
>
>> Since they're windowless, they are not EPROM (remember what the E
>> stands for), but plain ROMs.
> Nope.  I meant exactly what I wrote.

[...]

Good point about it being the same chip. I hadn't considered that 
possibility. I know that for some 27-series proms, there were certainly 
both mask programmable as well as EPROM versions, where the mask 
programmable was more persistent safe. EPROMs have a risk of loosing 
their content eventually, even if not exposed to UV light.

> As for which signal you use for what - it doesn't matter.  OE puts the
> chip into a low power state just as effectively as CS - assuming that
> the part isn't in programming or ID mode.  Since the part is never
> written (in the terminal), this effectively gives you 2 CS pins
> (effectively ANDed), and thus decoding requires at most an inverter.

Not entirely true.
OE should timing wise be done after CS and addresses have been stable 
for a certain time. And power consumption of the chip is related to the 
control of CS, and is not related to OE.

While power consumption might not be a problem, and the timing can be 
solve, it does mean that driving CS and OE cannot be done identically. 
If you use OE as a CS, you should make make sure the address is stable 
some time before you activate OE, and if you use CS, you need to still 
drive OE at a point later in time, and not just tie them together or 
something.

> The 27C256 is a 32K x 8 part; it has no A15 (but the cartridge socket does.)

Yes, that was obvious.

> Keven pointed out that the odd chip is probably the character generator
> ROM - thus the separate address and data bus - and it doesn't need a CS
> or OE.  It's always reading something.
>
> As I've written before, rather than guessing, a few minutes with an
> ohmmeter can sort all this out.
>
> I'm leaving that - and further exploration - as an exercise to the reader.

I seriously doubt it's a character generator ROM in the normal sense of 
the word. The VT340 do not generate character output in hardware.
It's a graphic terminal, which stores the text in the the bitmap, as far 
as I remember (I seem to remember being able to go into graphics mode 
and affect text already written). Also, you have soft definable 
characters, so the CPU need to have access to the same memory the 
character generator would use anyway, and it has to contain some RAM, 
minimum. So it needs to be in the normal memory space of the CPU.

But there is indeed two address and databuses, so I think it's fair to 
say the two select lines are only used for a subset of the PROMs.

There might be data in one ROM that is copied into RAM at startup. 
Character definition tables, for example, I could imagine.

Anyway, most things can be worked out my doing the measurements you 
suggest, yes.

	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