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.