🚀 go-pugleaf

    In response to Timothy Johns and Al's posts about ROM paks:

    Tandy used a variety of varient techniques for making ROM paks.
This in part because the cost of available technologies changed,
and perhaps more because Tandy in many cases just contracted withi
a given company to supply them with, say, 20,000 copies of their
game, and the company got to decide how to make the ROM pak, with
a result that different companies choose differnt minor varients of
how to do this.

   Some Tandy ROM paks used 2764 or 27128 EPROMs.  I just picked
up copies of Monster Maze and Dragon Fire at a local flea market
that were of that sort.  (I bought them for $1 each, and was easily
able to desolder the EPROMs, resulting in my having two customizeable
ROM paks that took standard 28 pin JDEC DIP type EPROMs.  The circuit
boards even had a trace on them for A14, allowing me the option
of modifying them with one trace cut and two jumper to allow them
to accept a 27256 EPROM)

   Many use masked ROMs.  Mostly those that were pin compatible
with 2K by 8 2716's, 4K by 8 2732's, and the 24 pin 8K by 8 68766.
Rom paks that used a single 28 pin ROM or EPROM were actually
relatively rare.   I have one copy of the Tandy Service Diagnostic
Tests ROM pak for the CoCo 1 that used FOUR 2716 EPROMs (apparantly
when it was made 2764's either did not exist or were very expensive).

   Some of the last ROM paks made used 32K or bank switched 64K or 128K by 8
masked ROMs.   I think it was Predator that used a 28 pin 128K by 8
(one megabit size) masked ROM.  Note that there is NO EPROM that is
equivalent to a 28 pin 1 megabit masked ROM... 1 megabit EPROMs
of the JDEC series are all 32 pin parts.  As I noted, since the CoCo 1 and 2
can address no more than 15.75K worth of external ROM space, and
the CoCo 3 can address no more than 31.75K (there may be a snekay
way to address a full 32K... ARTFLEXSER can answer that one), any
effort to use more than 32K of ROM space requires some sort of
"mini memory manager" circuit on the ROM pak.  Typically this consists
of a latch chip that is clocked by the *SCS line whose data inputs
are the data from the system bus, so that writing a byte to $FF40
acts to switch which bank of the EPROM is selected, for the outputs
of the latch (one or two or more as needed) are used to control
the high order address lines on the ROM chip... A15 and up.

   And now to those ROM paks that, when you open them, don't have
any ordinary DIP chip, but rather have a little blob of epoxy
with traces coming out from it:   Those are utilizing what is
called "COBS" technology.   COB stands for Chip On Board.
Instead of bonding the silicon to a carrier and encasing that carrier
in ceramic or plastic to make an IC chip, the silicon die of
the masked ROM is bonded DIRECTLY to the printed circuit board, then protected b
 y
a littl e dab of EPOXY.


    I hope those of you equiring about ROM paks found this of interest.


  Oh...  to my knowledge, Tandy DID NOT offer a service of making
small production run ROM paks for people.   But other companies
at one time or another did.   Also, Tandy did make at least
one ROM pak for service work, and no doubt issued or received
from contractors prototypes of ROM paks for approval.   Perhaps
the one described that had a dot matrix printed lable on it might
have been one of those.   Or it might have been a pak that lost
its lable, for which someone MADE UP a lable.



   ---marty