The datasheet says to rest vpp at vcc. Most do, even if elsewhere they also say that writing is disabled as long as vpp is below vpp and the only low limit is pretty much the same as for any other pin, like 0.5 or 0.7 below vcc.
I never saw a reason why, but it's definitely a consistent suggestion in datasheets to hold vpp at vcc rather than gnd. Maybe holding it at gnd wastes a little power or stresses a gate somewhere through reverse leakage and vcc is closer to where that gate is when running. So, it may still be true that any voltage from gnd to vcc is legal and functional, yet one is still better than the other. bkw On Mon, Nov 14, 2022, 12:19 AM Mike Stein <[email protected]> wrote: > Too bad I don't have a 200; you're probably right but since it's not > clearly defined in the data sheet I'll have to dig out one of my other > systems using a 2764 and check it for myself ;-) > > But I have to ask: why pull pin 27 to Vcc? What happens if you pull it low? > > On Sun, Nov 13, 2022 at 9:06 PM Brian K. White <[email protected]> > wrote: > >> 27C64 can't be used as-is. >> pin 27 is A15 and needs to needs to go to a non-existing 2nd OE or CE. >> >> You'd need an adapter board that combines pin 27 (A15) from the socket, >> with pin 22 /CE0 (/BANK#1) from the socket, to produce a single /CE to >> pin 22 of the chip. >> >> You could do that with a single 2-gate NAND part the same as in this >> model 200 RAM >> https://github.com/bkw777/TANDY_200_RAM#single-bank---sop >> >> https://raw.githubusercontent.com/bkw777/TANDY_200_RAM/main/TANDY_200_RAM.svg >> >> That is using one nand gate just to invert /CS1 to CS1, >> then ANDing those to make a single CS, and inverting that to make a >> single /CS output to the chip's /CE >> >> The point is getting the job done with only 2 gates of the same type so >> it's all from a single part. >> They even make little 8-leg parts with just the 2 needed gates. >> >> For 27C64 you'd need to do that with /CE0 from bus pin 22 and CE1 from >> bus pin 27, to produce a single /OE to chip pin 22. And chip pin 27 >> pulled to VCC by anything from 100k to a plain trace. All other pins >> including /CS would go 1:1 socket to chip. >> >> >> On 11/13/22 19:38, Mike Stein wrote: >> > Duh; shoulda looked more carefully; it really IS A15 (not A13) and it >> > goes to /CE1 (or CE1); double duh! Thanks, Brian. >> > >> > So it looks like the only non-standard pin is pin 27, since 26 is not >> > used. Has anyone actually tried a JEDEC standard 27C64? >> > >> > Pin 27 is the Program pin which looks like it might effectively be an >> > active high OE (since there is no program voltage. >> > >> > m >> > >> > >> > >> > On Sun, Nov 13, 2022 at 5:27 PM Brian White <[email protected] >> > <mailto:[email protected]>> wrote: >> > >> > 8k. >> > >> > HN61364 = 8k >> > A15 is connected to a chip select to enable/disable the whole chip >> > (really just output enable not chip enable despite the /CEx labels), >> > and the actual address lines are only A0-A12 >> > >> > It's mostly like 27C64 but with 3 OE lines, customer programmable to >> > be either active high or low as part of the mask programming. >> > Although the schematic labels them as /CE0 /CE1 /CE2, really they >> > are all OE not CE, and it appears that /CE1 should be shown as >> > active-high, so really: /OE0, OE1, /OE2 , and /CS is a normal actual >> > whole chip enable, active low. >> > >> > I was just now in the middle of drawing up an adapter for that chip >> > like FlexROM_102 but for that chip, to facilitate using the main rom >> > replacement feature of REX Classic in a 200 the way I have already >> > for 100 and 102. >> > >> > But it may be simpler to make a different kind of adapter that >> > replaces both chips with a single larger chip on a single adapter in >> > the main rom socket and simply remove the 8k chip. The /BANK1 line >> > goes to both chips the same, and A15 ends up activating one or the >> > other exclusively at any given moment. >> > >> > bkw >> > >> > On Sun, Nov 13, 2022, 4:48 PM Mike Stein <[email protected] >> > <mailto:[email protected]>> wrote: >> > >> > Looking at the schematic, are you sure it's 8K and not 16 >> (27x128)? >> > >> > Looks standard except for pins 26 & 27 >> > >> > On Sun, Nov 13, 2022 at 4:23 PM Stephen Adolph >> > <[email protected] <mailto:[email protected]>> wrote: >> > >> > Georg, >> > What type of ROM chips did you use, when you replaced your >> > ROMs with patched versions? >> > I've been pondering what the simplest way to do that is. >> > The 8K M13 socket is wired oddly, and doesnt seem compatible >> > with a 27C64. >> > thx >> > Steve >> > >> > On Sun, Nov 13, 2022 at 12:30 PM Georg Käter >> > <[email protected] >> > <mailto:[email protected]>> wrote: >> > >> > Hello together, >> > >> > I own a M200 "German/EU Version" (Art.Nr. 26-3860H) >> > w/modem, for German market it was delivered with set of >> > keyboard >> > >> > caps and a data tape including driver for keyboard and >> > printer. I tried this in VirtualT and it seems to work >> > so far. To run REX >> > >> > on my M200 I replaced original ROM by ROM from VirtualT >> > patched to serve german keyboard mapping. >> > >> > For your reference I´ve added original ROM files and >> > files from tape for your reference. >> > >> > Regards >> > >> > Georg >> > >> > >> > Georg Käter >> > Gangolfsweg 44 >> > D-52076 Aachen >> > Tel. : +49 2408 7194987 >> > Fax. : +49 2408 7196758 >> > Mobil : +49 171 4839954 >> > E-Mail : >> > >> > [email protected] >> > <mailto:[email protected]> >> > >> > ========== Ihre Nachricht >> > ========================================== >> > >> > *von* : Cedric Amand <[email protected] >> > <mailto:[email protected]>> >> > *gesendet* : Sonntag, 13. November 2022, 15:37 >> > *an* : [email protected] >> > <mailto:[email protected]> >> > *Betreff* : [M100] custom key mapping generator for >> > Tandy 200 >> > >> > __________ Originalnachricht >> > _______________________________________ >> > >> > My point exactly Brian ! >> > How did they come up with that idea ? It makes no >> > sense. It really prevents you from using the option >> > rom socket. >> > The docs does not talk about removing it. >> > And even if you could remove it ; the installation >> > procedure of that ROM is not easy at all, requires >> > to type two "calls" with the freaking keyboard >> inverted. >> > OK - us nerds 40 years later can do it easily, just >> > type "CQLL", but imagine explaining that to a random >> > journalist in 1984 ?! >> > Especially as the french doc (which I happen to >> > have) says to type "CALL" not CQLL. >> > I also wonder if other markets are affected by this >> > plague, >> > If anyone here lives in germany and owns a qwertz >> > (or other keyboard variant) of the M200 : do you >> > have a "stock option ROM" as well ? >> > I also wish to thank Stephen publicly for the time >> > he invested into helping me, as indeed, you can't >> > use an option ROM (and even less a REX#) in those >> > non-qwerty M200s, and I think this research might >> > help some other people at some stage (this hobby is >> > booming right ? :) ) >> > We're (and I am) in the process of replacing the >> > main rom + 8KB rom with a 27C512 flashed with a >> > custom "native Azerty" firmware >> > Which should free up to option socket, for a REX# >> > I also plan to make other modifications to that >> > custom ROM, but we'll see if I get there. >> > I've also been experimenting in the past with custom >> > firmware for the my M102 for different reasons. >> > I'm a "modem" nerd and I have all the equipment >> > (PABX, etc) to make voice calls between my vintage >> > laptops - so it's important for me to have my modems >> > work. This required a custom firmware to make my >> > M102 work, with modem, with a REX#. ( OK I think >> > this kind of stuff is only relevant to me this time >> > :) :) >> > Le 2022-11-13 14:53, Brian White >> > <[email protected] <mailto:[email protected] >> >> >> > a écrit : >> > >> > Nice. >> > So the point would be to make the main rom >> > natively azerty to match the hardware, free up >> > the option rom slot for normal use, without >> > otherwise changing the main rom so that it >> > becomes incompatible with application software? >> > I guess you might even be able to make a dvorak >> > version and move the keycaps around? >> > I'm just trying to imagine the sales pitch for >> > that azerty 200 that needs the option rom, thus >> > preventing the use of any other option rom (or >> > at least making it pretty inconvenient by having >> > to swap them on every reset I guess?) >> > "Here's your new model 200. It's only half as >> > useful as others with no modem and no option rom >> > but you can still pay full price please." >> > -- >> > bkw >> > >> > On Sun, Nov 13, 2022, 8:28 AM Stephen Adolph >> > <[email protected] >> > <mailto:[email protected]>> wrote: >> > >> > hi folks, >> > Thought I would share this work. It is a >> > spreadsheet for computing the keyboard table >> > in the T200 so you can make native custom >> > keyboards for T200. >> > Why? >> > The AZERTY keyboard in Europe was >> > accommodated using an option ROM that kinda >> > hacked the keyboard. Keystrokes get >> > intercepted and corrected to be AZERTY even >> > though the main ROM is set up for QWERTY. >> > An alternative is to have the main rom >> > directly support AZERTY. >> > To do this, there are 6 keyboard mapping >> > tables that start at 9763h. Each table are >> > 44 bytes long. >> > This spreadsheet lets you assign the ascii >> > codes for each of the 44 affected keys, for >> > all 6 tables. (unshifted, shifted, GRAPH, >> > shift GRAPH, CODE, Shift CODE). >> > It is an excel spreadsheet that included the >> > analysis add it so that certain needed >> > functions are present. >> > Once you make the correct keyboard mapping, >> > the spreadsheet provides the 6x44 bytes in >> > assembly compatible form, so you can compile >> > and patch the tables with a hex editor. >> > This approach could be used for other >> > machines also. >> > Note - the AZERTY keyboard did NOT modify >> > the actual character set, so that is out of >> > scope. Of course it is possible to patch >> > the main ROM to change the bitmaps as well. >> > Not handled by this spreadsheet. >> > Comments welcome. >> > Steve >> > >> > >> > >> > __________ Ende Originalnachricht >> > __________________________________ >> > >> > *Vertraulichkeitsinformation: >> > *Diese Nachricht ist vertraulich. Die Informationen >> > dieser Nachricht sind ausschließlich für die persönliche >> > und vertrauliche Verwendung durch den/die oben genannten >> > Empfänger bestimmt. Wenn Sie kein beabsichtigter >> > Empfänger sind, bitte lesen, kopieren und verwenden Sie >> > die Nachricht nicht. Machen Sie sie nicht anderen >> > zugänglich. Bitte informieren Sie uns umgehend über den >> > Zustellfehler und senden Sie die Originalnachricht >> > per E-Mail an uns zurück. >> > >> > *Confidentiality Notice: >> > *This message is confidential. The information contained >> > in this message is intended only for the personal >> > and confidential use of the recipient(s) named above. If >> > you are not the intended recipient, please do not >> > read, copy, or use it and do not disclose it to others. >> > Please inform us immediately of the delivery error >> > and return the original message to us via e-mail. >> > >> >> -- >> bkw >> >>
