[Simh] DEC VT emulators on MAME

Tue Apr 18 13:06:58 EDT 2017

A quick look at the schematics around the connector for this card, there
are two sets of address/data lines. I'm guessing that one of the chips is
for the character rom,and the other 4 are for the program. I haven't looked
closer (using a tablet that's really painful with PDF's), so not 100%
certain of the split.
Thus the CS/OE is sufficient to handle the 4 program chips, and the other
doesn't need any chip .selection lines.

On Tue, Apr 18, 2017 at 9:12 AM, Timothe Litt <litt at ieee.org> wrote:

> 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
> <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 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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