7 Feb
2001
7 Feb
'01
4:22 a.m.
Ross Archer skrev:
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(a)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:
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.
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
**END QUOTE**
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
IEEE-488
sales and it was a strain on the cable manufactuer. IEEE-488
cables
nowadays run from $40-$90 new, of course it is a standard 'get'
item on
my mental thrift store shopping list.
--
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