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Notes on Memory Expansion for the Tandy 4000
The Tandy 4000 came with 1 Meg of memory (the LX with 2 Meg) as
standard, but many users want to expand, preferably without paying Tandy
prices. Since the booklet that came with the machine says almost nothing
about adding memory, there is some confusion about what, why and how. These
notes are an attempt to answer some of the more common questions.
It should be stated up front that the author has not expanded his
own 4000 past the second meg. Most information that follows is drawn from
the Tandy 4000 Technical Reference Manual and the comments of TRS80Pro
Forum members. Where an item is in question, that is pointed out. Readers
with comments or additional information are invited to foward them to
70035,140.
WHAT
The 4000 has eight sockets on the motherboard for Single Inline
Memory Modules (SIMMs), plus a slot for a memory expansion adapter board
which can hold another 8 SIMMs. The SIMMs are simply miniture circuit cards
holding 9 memory chips each. Each chip provides one bit of an 8-bit byte
(the ninth is for parity checking). As the SIMMs are installed in sets of
4, the 4000 can retrieve 32 bits (4 bytes) from memory at a time. The
SIMM's size may be either 256Kb or 1Meg, producing memory increments of
either 1- or 4-Meg for each set installed.
The rules for adding memory are:
1) The motherboard may hold either one or two sets of SIMMs.
2) The motherboard must be filled out before the expansion card is used.
3) If used, the expansion card must be fully populated.
4) If types are mixed, the 1Meg SIMMs must go on the mother board.
This produces the following possible configurations:
Total memory 1M 2M 4M 4M 8M 10M 16M
Expansion adapt. - - yes - - yes yes
256K SIMMs 4 8 16 - - 8 -
1M SIMMs - - - 4 8 8 16
Note that there are two possible ways to get 4 Meg. Several Forum
participants have been told that with 1Meg SIMMs the motherboard must be
completely filled, making the 4 x 1Meg configuration invalid. A number of
others have reported that they were currently running precisely that
configuration in 4000s, LXs and DECstations. The Tech Manual says it is
permited, but there may be some variation of the hardware which does not
(the 4000 came as Rev "A" and "C" and there may be others). No one has yet
reported going to 10 or 16 Meg.
WHY
Besides the obvious advantages of having more memory, adding the
second bank of SIMMs will speed up the 4000's memory. DRAM chips have a
minimum time required between succesive acceses, specified in nano-seconds
(ns). As CPU speeds increase it becomes more difficult and expensive to
supply memory that can keep up. A common method to allow slower memory to
run in a high-speed machine is to insert 'wait states' of one or more CPU
clock cycles between memory accesses. This allows memory to recover, but
wastes system speed.
The stock 4000 uses this approach if there is only one bank of
SIMMs. When the second set is added, however, the machine assigns
successive memory locations to alternate banks. Since the most common
access to memory is to fetch the next sequential byte, each bank can
alternately recover while its partner is supplying the data.
(Actually, memory in the 4000 returns 32 bits at a time, so the
interleaving occurs at 4 byte intervals. The 4000 also uses Page Mode
Access to further improve performance.)
Interleaving also explains why the expansion adapter must have both
banks installed. The system will detect the presence of a second bank at
boot-up and automatically begin interleaving, so any expansion board must
be able to follow suit.
HOW
In its simplest form, adding memory only requires inserting
(carefully !) an additional set of 4 SIMMs in the empty sockets on the
motherboard and running SETUP to tell the CMOS storage about it. Going
beyond the second bank requires buying an expansion card from Radio Shack,
populating it with two sets of SIMMs and inserting it in the dedicated
slot.
The SIMMs' speed is specified as 100-ns for the 4000 and 80-ns for
the faster LX, but 80-ns is probably a good choice for either. The price
difference is minor and the faster memory provides insurance against a
manufacturer who may have 'pushed' the rating a little. They must have 9
chips (SIMMs for the Mac only have eight) and should be surface mounted
(socketed chips would crowd the available space and interfere with
cooling). There are 30 pins on the SIMM board itself.
The 1991 Tandy catalog lists:
25-5029 Memory expansion adapter $ 140
25-5031 1 MB: 4ea 100-ns 256K SIMMs 300
25-5033 2 MB: 2ea 100-ns 1M SIMMs 500
25-5131 2 MB: 8ea 80-ns 256K SIMMs 500
25-5132 4 MB: 4ea 80-ns 1M SIMMs 1000
At this writing, mail order ads are quoting $25 ea for 256K SIMMs and $75
for 1-Meggers (i.e. $100 to add 1Meg or $300 for 4Meg). Several files in
the Forum libraries list dealers and many members will report their
experiences on request.
MISC.
Trivia from various threads:
At least two people have reported finding SIMM boards in their
machines with 3 chips in a row (rather than nine in parallel). These would
appear to be a pair of 4-bit x 256K chips and one 1 x 256K.
One Radio Shack salesman said the expansion board was no longer
available! He also said that you couldn't add memory without it. I do not
predict a long career for him.
The Tandy catalogs have at various times given different
descriptions for the same stock number (25-5131) and used two numbers for
the same item (the expansion board). A little caution may be in order when
reading the selection chart and descriptions.
The DECstation 316 is actually a Tandy 4000, but rumor has it that
DEC first debugged all the PLAs and ROM. If you were frustrated by the
Floppy Disk Controller jumper when you installed your dual controller
board, you might want to get friendly with a DEC techician.