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C-128 VDC RAM UPGRADE
Copyright 1986
Twin Cities 128
P.O. BOX 4625
Saint Paul MN 55104

This article by John Kress appeared in the October issue of Twin Cities 128
Permission is hereby granted to reprint provided the following conditions are
met:   1. You are not reprinting this in a profit-seeking publication
          or distributing it for profit
       2. The above copyright notice and these conditions appear with
          the article

The  very first time I had fired up my new C-128 I
loved the 80 column screen.  I remembered the days
of  my  Vic-20 with 23 characters per line.  Now I
had  a  computer  with  an  80 column screen and I
thought  to  myself  'Now  I own a real computer'.
Then  I came across a very basic 80 column graphic
program  that  used  a  joystick, and thought that
this is great, but it is too bad that you can only
use  one  color  on  the  screen  at  a  time, and
secretly  dreamed  of  owning an Amiga.  Then came
the day that I found out that the memory dedicated
to  the  Video Display Chip could be expanded from
the stock 16K to 64K of RAM.  Right then and there
I  was  hooked  (I  believe  it's  referred  to as
'Technolust'  in  these parts) and wanted to learn
all  I  could  about  expanding  the  RAM  to  its
maximum.

I should explain a bit about the VDC and it's RAM.
The  80  column  screen  uses  a separate chip for
screen  processing,  and  that  chip  has  its own
dedicated  RAM.   Unlike  the VIC chip, for the 40
column  screen,  which  uses part of the BASIC RAM
for  screen  memory,  the  VDC has 16K of RAM tied
into it that cannot be accessed in the normal peek
and  poke  method.   The  VDC  is  in  some ways a
separate  processor  that  operates  independently
from the rest of the C-128, and is able to work at
a  faster  clock speed than the VIC chip.  That is
the reason why the video display goes blank on the
40 column screen when the FAST command is issued.

Why expand the VDC memory?

Since  my  major  computer interest is GRAPHICS, I
wanted  to  find  out how things might be improved
with  more  memory.   One  of  the  first things I
pondered  was  the  graphics screen with attribute
RAM  for  color.   That  would allow more than one
color on the screen; although you're still limited
to  just  one color in any 8*8 pixel area, you can
have  more  that  one  color.   Then  there is the
ability  to  have  a combination of screens in VDC
memory,  up  to  4  graphic screens, up to 13 text
screens,  or a combination of both.  There is also
the  possibility  of  making  up  different  fonts
(print  styles) in memory and being able to change
them.


(Editor's note: The execution of this process will
void   your   computer's  warranty.   Readers  who
attempt  this  process  do  so  at their own risk.
Neither  this  author or Twin Cities 128 will take
responsibility  for  losses  due  to the attempted
execution   of   the   process  outlined  in  this
article.)

The parts you will need are :

*  Two  (2)  18 pin IC sockets, to be used to hold
the  RAM  chips.   These  can be found at most any
electronic supply house.

*  Two (2) 4464 Dynamic RAM Chips, with a speed of
120  ns.   Yes  that's  correct, 4464 RAM chips, I
know  that  the  Commodore  Programmers  Reference
Guide  says  4164  RAM chips, but 4164 chips are a
sixteen  pin  type.   The 4416 chips are an 18 pin
type  so  if you try to use the 4164's you'll have
two extra holes and nothing will work right.

The  4464's  can be a hard item to locate, as most
of the manufacturers are either discontinuing them
or  require  minimum  orders  and are reluctant to
release 'just' two.  If you have some friends or a
local  computer  group  or  club, get together and
purchase a larger quantity, you may have more luck
with a volume purchase.

You will need the following tools:

Wire cutter or nippers.

A small pencil type soldering iron (35 watts is be
enough, much more may get too hot.)

Some rosin core solder or better yet,
Solid  core  solder and a paste type non-corrosive
flux.

Small phillips screwdriver.

Small pair of pliers.

Now you're ready to begin:

NOTICE:    Integrated   Circuits   are   extremely
sensitive  devices,  and  very sensitive to static
charges.   A  small  static  shock  to  a IC is as
deadly  to them as a bolt of lightning would be to
a  human.   Take extreme caution to avoid a static
discharge to the computer.

First  thing  to  do is remove all power cords and
serial  connectors  from  the  C-128.   This means
everything,  since  the main board must be removed
from the machine.

Turn  the  computer over and locate and remove the
six  (6)  screws  holding the computer case halves
together,  then  carefully  separate  the  halves.
This  is  critical, because the keyboard and power
light  connector along with a ground braid for the
keyboard are still connected and could be damaged.
Carefully  remove  the  screw  holding  the ground
braid,  along  with  the  connectors for the power
light  and  keyboard,  taking note of their proper
connection.   I  found that the keyboard connector
was  installed  with  a  brown  wire closer to the
power switch, but things may be different on other
machines.

The next step is to remove the screws holding down
the  computer  board and the ground casing.  There
were  six  screws around the perimeter of the main
board/shield  assembly, remove all of these.  Then
you  must  use  the pliers to gently bend the tabs
holding  the  upper  ground  casing  to  the lower
casing.   There  is  one more screw mounted on the
ground  casing  in about the top middle, this must
be removed also.

There  is  one  spot  where the casing halves were
soldered  together  on my computer and that was in
the  front  right  side,  near  where  the numeric
keypad  sits.   Carefully  unsolder the halves and
lift  the  top half away.  What you now see is the
main  board with a small metal box on it, near the
center  of  the  board.  The chips that need to be
replaced are inside that metal box.

Inside the box are both of the screen controllers,
the  8564  Vic chip and the 8563 VDC chip.  Remove
the  lid  to the box and locate the two chips that
are  identified  on  the  circuit  board  with the
marking U23 and U25.  On my board these chips were
labeled  with  a  large  F  and  the  part  number
MB81416-12.   The  number  may  vary  due  to  the
manufacturer,  but  the markings on the board will
identify the correct chips.

Now for some very delicate surgery.  Since the RAM
chips  will  be  discarded,  I  suggest  that  you
carefully  cut away the chip pins (leads) from the
body  of  the  chip.   This will make removing the
remaining portion a lot easier, and leaves less of
a chance of damaging the main board than trying to
remove  the  whole chip, intact.  The next step is
to  carefully  unsolder  the pins from the circuit
board.   Make  sure  that you are working with the
correct  area,  as  things do get rather confusing
when you turn the board over and notice all of the
many  soldered  areas.   You  might be best off to
mark the correct area with a felt tip pen to avoid
confusion.

The  next  step  will be to install the IC sockets
into  the  circuit board.  Try fitting the sockets
into the board to make sure that they will fit all
of  the  way  through  the holes and that there is
enough  material  poking through for a good solder
joint.   Also  be  sure to orient the sockets with
the  small identification notch on the correct end
of  the  board.  The notch is printed on the board
so you should have no trouble.

When  you  have  the  sockets installed correctly,
lightly  bend  the pins over to hold them in place
and  you're  ready to solder them in place.  Again
be  careful, as drips of solder can cause a short,
and   a  cold  solder  joint  will  cause  a  poor
connection.

Now  you're  ready to install the 64K chips in the
sockets,  carefully inspecting the pins for proper
alignment.  Most of the time chips will be made so
that you will have to bend the pins inward, toward
the  center,  to  fit properly in the socket.  But
too much bending will break off a pin.

Once  you've  installed  the chips you're ready to
start  the  reassembly  process,  just reverse the
disassembly  steps  and  make  sure  to solder the
ground  case  halves again.  Reconnect your cables
and power up the computer.

If  all went well, you should see NOTHING UNUSUAL.
The  same  old  cold  start up message and nothing
different.   Some  of you might notice a different
pattern  on  the  screen  when  the initialization
process  is going on but after that you should see
the  same OLD SCREEN.  The cold start routines for
the  C-128  initialize  the VDC registers, and the
VDC  chip  is told that it still has the 16K chips
in  it.   In order to tell the VDC you've upgraded
the  RAM,  you  will have to set bit 4 of register
28.  When  this  is  done,  for  some  reason, the
character  information  gets scrambled, but a call
to  the  COPY  ROM  routine  at  $CE0C  clears the
problem.

If  your screen does not appear the way is should,
look  for a cold solder joint or a pin that is not
installed into the socket correctly.  I found that
when I tried to remove the old chips in one piece,
I  had damaged a portion of the copper foil on the
circuit  board  and  had to trace the circuits out
and  install a jumper wire from the 8563 socket to
one of the RAM sockets.

I  hope  to  be  passing  more  information to you
regarding  the  VDC chip and some programming tips
in the coming months, so keep looking.