From: brain@garnet.msen.com (Jim Brain) Newsgroups: comp.sys.cbm Subject: PC's as 64 HD's and the 6522 chip! Date: 7 Apr 1994 13:48:35 GMT Right after I posted about hooking a 6522 VIA up to a PC parallel port to turn the PC into a "char-banger" instead of a "bit-banger", someone posted that they had thought the 6522 had a problem with the shift register. The poster also said that Jim Butterfield had alerted him and others to it. Well, I have been in correspondence with "The Commodore Man", so I asked him to elaborate on the topic. Here is the two responses: > Yes, it's true. Although I didn't get official confirmation of this >long after, when a Spectrum article quoted the designers. > As you know, the first Commodore computers used the IEEE bus to connect to >peripherals such as disk and printer. I understand that these were available >only from one source: Belden cables. A couple of years into Commodore's >computer career, Belden went out of stock on such cables (military contract? >who knows?). In any case, Commodore were in quite a fix: they made >computers and disk drives, but couldn't hook 'em together! > So Tramiel issued the order: "On our next computer, get off that bus. >Make it a cable anyone can manufacture". And so, starting with the VIC-20 >the serial bus was born. It was intended to be just as fast as the >IEEE-488 it replaced. > Technically, the idea was sound: the 6522 VIA chip has a "shift >register" circuit that, if tickled with the right signals (data and clock) >will cheerfully collect 8 bits of data without any help from the CPU. >At that time, it would signal that it had a byte to be collected, and >the processor would do so, using an automatic handshake built into the >6522 to trigger the next incoming byte. Things worked in a similar way >outgoing from the computer, too. > We early PET/CBM freaks knew, from playing music, that there was something >wrong with the 6522's shift register: it interfered with other functions. >The rule was: turn off the music before you start the tape! (The shift >register was a popular sound generator). But the Commodore engineers, >who only made the chip, didn't know this. Until they got into final >checkout of the VIC-20. > By this time, the VIC-20 board was in manufacture. A new chip could >be designed in a few months (yes, the silicon guys had application notes >about the problem, long since), but it was TOO LATE! > A major software rewrite had to take place that changed the VIC-20 >into a "bit-catcher" rather than a "character-catcher". It called for >eight times as much work on the part of the CPU; and unlike the shift >register plan, there was no timing/handshake slack time. The whole >thing slowed down by a factor of approximately 5 to 6. > There's more (the follow-on C64 catastrophe), but that's where it >happened. --Jim And the saga continues ... > When the 64 came out, the problem VIA 6522 chip had been >replaced by the CIA 6526. This did not have the shift register problem >which had caused trouble on the VIC-20, and at that time it would have >been possible to restore plan 1, a fast serial bus. Note that this would >have called for a redesign of the 1540 disk drive, which also used a VIA. > As best I can estimate - and an article in the IEEE Spectrum magazine >supports this - the matter was discussed within Commodore, and it was >decided that VIC-20 compatibility was more important than disk speed. >Perhaps the prospect of a 1541 redesign was an important part of the >decision, since current inventories needed to be taken into account. > But to keep the Commodore 64 as a "bit-banger", a new problem arose. >The higher-resolution screen of the 64 (as compared to the VIC-20) >could not be supported without stopping the CPU every once in a while. >To be exact: Every 8 screen raster lines (each line of text), the CPU >had to be put into a WAIT condition for 42 microseconds, so as to allow >the next line of screen text and color nybbles to be swept into the chip. >(More time would be needed if sprites were being used). > But the bits were coming in on the serial bus faster than that: aD >a bit would come in about every 20 microseconds! So the poor CPU, frozen >for longer than that, would miss some serial bits completely! > Commodore's solution was to slow down the serial bus even more. >That's why the VIC-20 has a faster serial bus than the 64, even though >the 64 was capable, technically, of running many times faster. > Fast disk finally came into its own with the Commodore 128. --Jim Now someone also told me at one time that they had seen a fastloader that same someone said he thought it was odd that the author of the loader had credited Commodore with the routines. Well, I can hazard a guess that the routines were the ones they had wanted to put in the 6522, but had to scrap due to the 6522 problem. Now I have no idea what the problem is/was, but I am eager to find out. However, I rescind my plans to build something around the 6522 until we find out what the problem is. Jim "Just the Facts" Brain -- Jim Brain, Embedded Systems Designer, Brain Innovations. brain@msen.com Dabbling in VR, Old Commodore Computers, and Good Times! "The above views DO reflect my employer, since I am my employer" - Jim Brain -- From: dillon@apollo.west.oic.com (Matthew Dillon) Newsgroups: comp.sys.amiga.advocacy,comp.sys.cbm Subject: Re: COMMODORE STOCK SPECULATION!! Date: 8 Apr 1994 23:15:06 -0700 In article <1994Apr8.020037.9386@pimacc.pima.edu> ppugliese@pimacc.pima.edu writes: :In article <2o1pjq$20a@netaxs.com>, firefoot@netaxs.com (Ivan Kohler) writes: :> ppugliese@pimacc.pima.edu wrote: :> : In article <2ns7bt$ivu@falcon.ccs.uwo.ca>, koops@gaul.csd.uwo.ca (Luke :> Koops) writes: :> : > In article <1994Apr3.181524.821@mnemosyne.cs.du.edu>, : :> : > :> : > Grapevine has made KS chips and 8372A's. If you are in good enough with :> : > Commodore, you can make your own chips with an eprom burner. We used to do :... : : :No. I was only referring to the chips that the previous poster mentioned. :The custom ships would have to be fabricated, although the CIA may not :be so hard to find, after all. I say that because I once was looking for : the VIA chips that the 1541 uses & found them "on the rack" in an elec- : tronics supply house. Some guy in the LUG said that those chips were used : for "alot of things". Most of Commodore's older chips are also made by Rockwell, but unfortunately they haven't fixed any of the bugs. For example, the 65C22 still has a number of interrupt/timer race conditions. I don't quite remember the details of all the bugs, but I found on the order of 3 hardware bugs in the 65C22. The chips used in the C64 also have problems... Bryce found at least one hardware race condition that would screw up a timer interrupt. I do not know whether any of Commodore's newer chips have secondary sources... certainly the big custom chips do NOT. But even if C='s chip fab goes out of business, they could easily license the schematics to a professional workshop. NRE would probably be around $50,000 to reimplement them with a more modern process (Commodore was using 5 micron NMOS last I heard, or is it 3 now?). -Matt -- Matthew Dillon dillon@apollo.west.oic.com 1005 Apollo Way Incline Village, NV. 89451 ham: KC6LVW (no mail drop) USA Sandel-Avery Engineering (702)831-8000 [always include a portion of the original email in any response!]