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 <[email protected]> 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:[email protected] > <[email protected]>] *On Behalf Of *Timothe Litt > *Sent:* Tuesday, April 18, 2017 10:42 AM > *To:* [email protected] > *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, [email protected] 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. > > > > > > _______________________________________________ > > Simh mailing list > > [email protected] > > http://mailman.trailing-edge.com/mailman/listinfo/simh > > > > > > _______________________________________________ > Simh mailing list > [email protected] > http://mailman.trailing-edge.com/mailman/listinfo/simh >
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