Congratulations on "solving the mystery". Of course there never really was a mystery.
I said as much , although not in so many words and without confusing the issue with irrelevant references to /WE and voltage levels and confusing Vpp with /PGM. It only needed confirmation by someone plugging in a 2764 that PGM overrides /OE, since it isn't defined in the datasheet, in case the particular unit in question had a different motherboard. >From my posts: "Pin 27 is the Program pin which looks like it might _effectively be an active high OE_ (since there is no program voltage." and "if /OE and /PGM disables data out then a 27C64 should work." To which you replied that "27C64 can't be used as-is", "pin 27, being /WE (/PGM), still needs to be held at VCC" etc. To be followed by a tedious thread of me suggesting that someone with a 200 actually try it, and you arguing at length that it couldn't work. Glad to see that you managed to solve the mystery all on your own. I think in future I'll refrain from wasting time with technical comments and leave it to "experts" like you. m . ----- Original Message ----- From: "Brian K. White" <[email protected]> To: <[email protected]> Sent: Thursday, November 17, 2022 1:30 PM Subject: Re: [M100] custom key mapping generator for Tandy 200 > > > > On 11/14/22 11:47, Georg Käter wrote: >> Hello together, >> >> my Tandy 200 (European version, build in 1985) uses main PCB >> #-PLX178AHIX. This board accepts >> >> standard UV-eraseable EPROM 27C256 (32kB) and 27C64 (8kB) 150ns type w/o >> any modification. >> >> Maybe there are other revisions of PCB with different EPROM pinout I´m >> not aware of. > > In a conversation off-list just now I think we just figured out why it > works. > > I'll just describe verbally but to follow along and check the work, the > reference material you'll need are the service manual for 200 to get the > schematic for the 200, and the datasheets for HN61364, and 27C64. All > easily googled. And in the 200 schematic you're looking at the M13 and > M15 positions (top-right corner), and one gate on M29 that combines 2 > signals into one pin on M15. > > To start, all else being equal, > * both chips get their /CS at the same time from the same /RD line from > the 200 > * both chips, (in a sense, if you count the OR gate on M29 as part of > M15) get their /OE at the same time from /BANK1 from the 200 > * A15 ends up interlocking the two chips so that when one is active the > other is inactive. When A15 is low, only M15 is enabled, and when A15 is > high only M13 is enabled. This assumes that the pin labelled /CE1 is > really an active high not an active low, and we CAN assume that, because > all other pins on both chips are not up for any kind of debate, they are > known and certain and not ambiguous. If pin 27 on M13 were actually > active-low, then both chips would be active at the same time. > > (The pin names on M13 are a bit confising because for some reason the > 200 schematic labels the 3 OE pins as /CE0 /CE1 /CE2. But they are > really OE pins not CE pins. The /CE pin is /CS, and for this discussion > I'm calling the others /OE0, OE1, and /CE2. OE1 is really active-high so > that's yet another correction.) > > So the tldr for the normal case with original parts is: > A15 low=M15 high=M13 > > Now with a standard 27C64: > * pin 1 is NC from the socket, which means VPP is floating. That is bad > in general, but you get away with it in this case because it's not a > normal input but a VPP that needs a much higher voltage than vcc or any > voltage any other pin ever sees. Since no pin anywhere on the chip ever > sees anything outside the range of gnd to vcc, then a flapping input may > flap, but shouldn't be able to float any higher than vcc, and vcc is > just fine. Ideally we want that pin nailed to vcc even. It means we can > essentially ignore VPP, and ignore the write implications of /WE. We > can't ignore /WE, but we can ignore the *write* implications. It can > never write, regardless what state /WE is in, regardless what state any > other pins are in. > * /OE has the same /BANK1 as the /OE on M15 (via M29) > * A15 is on /WE > > So, it's just down to answering: What happens when A15 is high, and when > it's low, when the states of the other pins are all VPP<=vcc, /CE low, > /OE low? > > A15 low -> /WE low, which is technically undefined behavior being low at > the same time as /OE, but apparently overrides /OE and prevents output. > /CE and /OE are both low so it wants to output data, but /WE blocks it, > and yet, /WE doesn't write either because of missing VPP. > > And, that is what we happen to want. Above in the stock case, A15 low = > M13 disabled. > > A15 high -> /WE disabled, /OE is low so the chip outputs, and again that > is what we want. Above, A15 high = M13 active. > > So /WE without VPP is kind of being abused into acting as a secondary > active-high OE, which is what the HN61364 has explicitly on that pin. > (well, it's not that HN61364 has pin 27 = active high OE1, it's that the > pin is configurable and the mask programming happens to have programmed > that pin to be active high in this case.) > > So, it's not merely seeming to work by luck and maybe it's just lucky > that you don't get data corruption because of chance and statistics, but > really working, like you will never get data corruption because the > logic rules don't allow for it. > > And it doesn't hurt the 200 electrically either. There are no outputs > fighting each other or anything like that. Electrically fine. > > Myself I would lift out pin 1 and bodge it to pin 28 on all 27xxx chips > put in place of mask roms, but it's not really necessary. > > Oh the HN61364 has one other non-standard pin, but you can ignore that > too. Pin 26 is a 3rd active-low OE, and tied to gnd in the socket, so > it's not used by the 200 and hard wired to just be enabled at all times, > meaning you can pretend it doesn't exist. And it's a NC pin on a 27C64, > so the gnd doesn't matter and so again you can ignore it. > > And that's that. I think. Mystery solved? > > -- > bkw > > >> >> >> 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* : Brian K. White <[email protected]> >> *gesendet* : Montag, 14. November 2022, 16:24 >> *an* : [email protected] >> *Betreff* : [M100] custom key mapping generator for Tandy 200 >> >> __________ Originalnachricht _______________________________________ >> >> On 11/14/22 10:11, Brian K. White wrote: >> >> On 11/14/22 09:12, Brian White wrote: >> >> 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. >> >> derp I mean the low limit is a little below GND or VSS not VCC. >> >> Although a Microchip 27C64 datasheet I happen to be looking at >> at the > moment does actually say the vpp read voltage is min >> vcc-0.7, max vcc. >> >> >> More derp, I obviously confused VPP with /WE there. >> But pin 27, being /WE (/PGM), still needs to be held at VCC not GND, >> although theoretically any writes should still fail without VPP, but >> still, you just hold /WE at VCC for reads. >> >> >> >> >> __________ 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
