C64 question

From: Lawrence Walker <lgwalker_at_look.ca>
Date: Wed Feb 7 12:22:50 2001

> Ross Archer skrev:
> >And if I find it somewhere on the 'net, how to get it into
> >the C64? Has anyone solved that one?
> >I know it's got an RS232 port, so perhaps I could write a small
> >BASIC program to bootstrap in the program at low speed and develop
> >something like an Xmodem download with the downloaded assembler. Just
> >wondering if anyone has a better way to get software from 'net to
> >C64, and knows a good source for same. Maybe a cave full of
> >cartridges like the Atari has exists out there... :)
> Try to have a look in ftp.funet.fi:/pub/cbm/crossplatform/transfer/.
> If you've really got an RS-232 interface, try a nullmodem and Over5
> (available both in Amiga and Linux versions, perhaps a Windows port,
> too), which has a short BASIC RS-232 transfer program listing so that
> you might download a real terminal emulator for further transfers. In
> other cases, there are a lot of cables for connecting the 1541 to
> every kind of computer out there, or as a last-ditch effort, wav-prg
> (Windows and Linux versions exist) which will output a T64 or binary
> file as tape signals for recording on datasette and loading on the
> C64.
 TMK the C64 serial port isn't really RS232 compliant. In searching
thru my archives in regard to Jim Brain I ran across this interesting
little historical tid-bit written by Jim Brain and quoting a neighbor of
mine Jim Butterfield, another seminal Commodore figure who was
also the on-screen tech-man for a mid-eighties TV show on
computers called "Bits and Bytes".

From: brain_at_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
to turn the PC into a "char-banger" instead of a "bit-banger",
posted that they had thought the 6522 had a problem with the shift
register. The poster also said that Jim Butterfield had alerted him
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
>computer career, Belden went out of stock on such cables
(military contract?
>who knows?). In any case, Commodore were in quite a fix: they
>computers and disk drives, but couldn't hook 'em together!
> So Tramiel issued the order: "On our next computer, get off that
>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
>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
>The rule was: turn off the music before you start the tape! (The
>register was a popular sound generator). But the Commodore
>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
>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-
>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
>register plan, there was no timing/handshake slack time. The
>thing slowed down by a factor of approximately 5 to 6.
> There's more (the follow-on C64 catastrophe), but that's where it

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
>which had caused trouble on the VIC-20, and at that time it would
>been possible to restore plan 1, a fast serial bus. Note that this
>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
>supports this - the matter was discussed within Commodore, and
it was
>decided that VIC-20 compatibility was more important than disk
>Perhaps the prospect of a 1541 redesign was an important part of
>decision, since current inventories needed to be taken into
> 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
>To be exact: Every 8 screen raster lines (each line of text), the
>had to be put into a WAIT condition for 42 microseconds, so as to
>the next line of screen text and color nybbles to be swept into the
>(More time would be needed if sprites were being used).
> But the bits were coming in on the serial bus faster than that:
>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
>That's why the VIC-20 has a faster serial bus than the 64, even
>the 64 was capable, technically, of running many times faster.
> Fast disk finally came into its own with the Commodore 128.


Now someone also told me at one time that they had seen a
same someone said he thought it was odd that the author of the
credited Commodore with the routines. Well, I can hazard a guess
routines were the ones they had wanted to put in the 6522, but had
due to the 6522 problem. Now I have no idea what the problem
I am eager to find out. However, I rescind my plans to build
around the 6522 until we find out what the problem is.

Jim "Just the Facts" Brain


  The 64 and VIC would have had a faster drive if it wern't for the
cables and the chip problems... Probably HP was doing major
sales and it was a strain on the cable manufactuer. IEEE-488
nowadays run from $40-$90 new, of course it is a standard 'get'
item on
my mental thrift store shopping list.
Reply to:
Received on Wed Feb 07 2001 - 12:22:50 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:33:44 BST