[Simh] DEC VT emulators on MAME

[Timothe Litt]

That part is easy - the high address (and bank) bits are probably
controlled by a hardware register.  Likely the lowest block (perhaps
8/16/32K) is not bank switched (so the bank switch code has a place to
live). 

The rest is switched by writing the desired bank into the register.  But
there's also a 27256 on the terminal board, which may be the static
block.  In that case, all the ROM cartridge is likely bank switched by
the register.

The part Malcom needs to figure out is how the 5 hardware chips are
organized into chip selects onto the 2 address and 2 data buses that
come out from the connector.  This is a separate issue from the logical
banking.

There are 2 bank selects and one excess address bit.  Encoded, that
would give you 8 chips.  But that would require decoding hardware on the
cartridge.

Without a decoder, each chip has a CE and and OE.  So if a bank select
line goes directly to 2 roms' common CS, the extra address bit can
control output enable.  That scheme can work for 4 ROMs.  But there are 5...

So either I missed a select line on the connector, or there's a decoder
on the ROM cartridge.

It's possible that the 5th ROM was a late addition (programmers always
need more code space), so an extra select line might have a different
name on the terminal control schematics.  (Renaming a signal can have
all kinds of ripple effects.)

Anyhow, I hope Malcom is successful.

On 18-Apr-17 10:59, Tim Stark wrote:
>
> Ok, I now get it.  Since that it total 160K ROM space, 8031/8051 can
> only access 64K space with 16-bit address lines. Let’s figure out how
> to access them  with bank select lines and write down which chip is on
> specific bank number.
>
>  
>
> Tim
>
>  
>
> *From:*Simh [mailto:simh-bounces at trailing-edge.com] *On Behalf Of
> *Timothe Litt
> *Sent:* Tuesday, April 18, 2017 10:42 AM
> *To:* simh at trailing-edge.com
> *Subject:* Re: [Simh] DEC VT emulators on MAME
>
>  
>
> 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.
>
> 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.
>
> 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.  It's not obvious how the 5th
> chip is selected - perhaps there is a decoder on the card.  A couple
> of NAND gates, or perhaps a decoder to decode the 2 bank selects?  I
> didn't backtrack through the schematics to find out how the selects
> are generated.
>
> In any case, some time with an ohmmeter should let you figure it out. 
> At worst you'd need 2 headers (1/bank), but most ROM programmers have
> strong drivers (address), and even 5 chips in parallel should be OK
> for the data bus.  So you can probably get by with 1, and some jumpers
> (or a dipswitch) to set A15/the BS for dumping each one.
>
> DEC ROMs should have a checksum (or more likely, CRC), so you can
> verify that you dumped them correctly.  This would usually be in the
> last byte(s) of each chip - except where booting starts at the highest
> address.  (E.g. some Intel CPUs).  Then look at the beginning :-)
>
> Have fun.
>
> On 18-Apr-17 09:25, malcolm at avitech.com.au
> <mailto:malcolm at avitech.com.au> wrote:
>
>     Just a quick update: I've broken open the VT340 ROM cartridge.  Inside are 5 x surface-mount N27C256 ROMs.
>
>      
>
>     Some pictures of the ROM cartridge are now included on this page -> http://avitech.com.au/?p=1818 
>
>      
>
>     Is there anyone who has the tools, time and interest to remove these ROMs and dump the contents?  If so, please let me know and I will pay the cost of shipping to get this cartridge to you.
>
>      
>
>     Malcolm.
>
>      
>
>      
>
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>  
>

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