Horizon Ramdisk


Blow-up of ramdisk board

Shown here is a 1.5 Megabytes Horizon Ramdisk 3000, with RAMBO board installed. All chips are socketed except on the RAMBO board.

Previous versions of this board may look differently. Also, different memory configurations are possible, and several modifications could be made (Phoenix, RAMBO, 32K).


PE-box connector

  • Connections #2 to 60 on this side, connections #1 to 59 on back side
  • 244: Two 8-bit buffers, 74LS244 for the address bus and control lines.
  • 245: A 8-bit transceiver, 74LS245 for the data bus.
  • 7805: Voltage regulator, UA7805, +5 volts. Three caps under the heat sink.


    • Backup power

  • Bat: Three NiCad batteries 1.2V, 0.18 AH. They power the SRAMs and the 74HC154 (which is why these are HC instead of LS).


    • CRU logic

  • DIP: An 8-switches DIP switch to select a CRU address between >1000 and >1700.
  • 138: Two 3-to-8 decoders, 74LS138 (one for the switch, one to select the proper '259).
  • 259: Two 8-bit addressable latch, 74LS259 for CRU output (16 bits).
  • T: A transistor NTX2N2222A to control the red LED.
  • T2: Another 2N2222, not mentionned on the schematics. Used to control the green LED ??
  • LEDs: A red light-emitting diode reacts to CRU bit 0. A green LED (barely visible over green background) is connected to the CS2 pin of the DSR RAM for test purposes.


    • Memory

  • SRAM: In this case, twelve HM628128LP-10 static RAM, 128 Kbyyes each (total = 1.5 megs).
  • DSR: A SRM2264C static RAM, 8 Kbytes.


    • Selection logic

  • 138: A 3-to-8 decoder, 74LS138 (under the RAMBO board).
  • 156: Half a dual 2-to-4 decoder, 74LS156. (Was under the RAMBO board, now on it).
  • SW: Main switch that prevents the DSR from showing up at >4000-5FFF.


    • Memory paging logic

  • 138: A 3-to-8 decoder, 74LS138.
  • 154: A CMOS 3-to-16 decoder, 74HC154.
  • U2b: Socket for another '154 to support 12 more SRAM chips, piggy-backed on the existing ones.
    Also used by the Phoenix modification.


    • Phoenix modification

    Mainly for use with the "Geneve". Splits the Ramdisk in two: a small boot drive controlled by 8 CRU bits, and a larger ramdrive controlled by 16 bits. When installed in a TI-99/4A such a modified board will appear as two Ramdisks having different CRU addresses.

    The board shown above has not been modified, but carries the necessary sockets so that the modification can easily be made.

  • U25: Socket for a 74LS00 quad AND gates. Two gates selects two CRU addresses, the other two are mounted as a flip-flop to count the number of CRU bits written (8 vs 16), i.e. select the drive.
  • SW2 & SW3: Room for two additional switches to toggle between "TI-99/4A" and "Geneve" modes.
  • The Phoenix modification also implies re-wiring some of the 74LS138 and using two 74LS154.


    • RAMBO board

    The RAMBO board lets you access part of the Ramdisk memory at CPU memory in the area >6000-7FFF.

  • RAMBO: A programmable logic array (PAL) that contains all the necessary logic gates.
  • 156: The RAMBO board is plugged into the socket meant for a 74LS156. For this reason, a new 74LS156 is present on the board.
  • The blue wires are the various connections of the RAMBO chip. One goes under the board, to a bent-out pin of the 74LS138 that lies under here.


    • 32K modification

    This modification adds some memory so that the board duplicates as a 32K memory expansion. It involves the following:

  • Piggy-backing a 32K SRAM chip on top of the DSR chip. HM62256-LP12 or 43256-12L.
  • Piggy-backing a 74LS08 quad-AND gates on top of the 74LS138 that's near the DIP-switch (just for power supply, no common pins).

    • Obviously, the board show here doe not carry this modification.


    For details, see my Horizon Ramdisk page.


    Revision 1. 8/26/01. OK to release.


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