[Simh] DEC VT emulators on MAME

Tue Apr 18 10:59:03 EDT 2017

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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