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Microbee Core boards

Static RAM Core boards

Overview

  • The first versions of this board use banks of 2KB Static RAM (SRAM) chips to provide a total of 8KB, 16KB, 32KB or 56KB program memory for the machine. Later versions also support 8KB SRAM chips.
  • SRAM Core boards can optionally be fitted with a battery (4-5 volts) that allows the RAM contents to be preserved while the machine is powered off. Later versions supported a rechargable battery.
  • Starting with support for 4KB EPROMs, later versions added support for 8KB and 16KB EPROMs as well as 8KB Mask ROMS.
  • All Static RAM Core boards provide for a 50 way Expansion Interface at the rear, though most do not have the associated connector fitted as standard.

 

Board Number Board Colour Component Side Image Solder Side Image
  • Notes
P/N 1632 REV D Blue or Green PN 1632 REV D   Component Side mb1632_rev_d_top PN 1632 REV D   Solder Side mb1632_rev_d_bottom
  • Used for the Microbee Kit.
  • RAM config is either 16K (8x 2KB SRAMs) or 32K (16x 2KB SRAMs).
  • Basic ROMs config is 4x 2532 4KB EPROMs labelled A,B,C,D.
  • EDASM/Wordbee option config is 2x 2532 4KB EPROMs.
  • Net option config is 1x 2532 4KB EPROM.
MB1732 Green mb1732_top mb1732_bottom
  • RAM config is either 16K (8x 2KB SRAMs) or 32K (16x 2KB SRAMs).
  • Basic ROM config is 2x 2764 8KB EPROMs.
  • PAK option ROM config is 1x 2764 8KB EPROM.
  • Net option ROM config is 1x 2532 or 2732 4KB EPROM.
  • The 3 wire modification is unknown.
MB1732-33 Green mb1732_33_04_bottom mb1732_33_04_bottom
  • RAM config is either 16K (8x 2KB SRAMs) or 32K (16x 2KB SRAMs).
  • Basic ROM config is 2x 2764 8KB EPROMs.
  • PAK option ROM config is 1x 2764 8KB EPROM.
  • Net option ROM config is 1x 2532 or 2732 4KB EPROM.
  • Provision for LED removed.
  • (C) Copyright 1983 by MICROWORLD now screen printed on top right corner.
MB1656 Green mb1656_top mb1656_bottom
  • Used for Microbee 64K, 64K Plus, Series 2 APC.
  • RAM config is 56KB (28x 2KB SRAMs).
  • Boot/Monitor ROM config is 1x 2532 or 2732 4KB EPROM.
  • A 50W expansion port allowed the board to be connected to an external floppy drive box that incorporated an FDC. Alternatively the boot ROM's monitor program could be used to load Basic/EDASM from tape.
  • The floppy controller was housed in an external drive box consisting of two 5.25" drives and connected to the Microbee using the 50W expansion bus. There were two controller types. One used the WD1793 and the WD1691 controller ICs (MB1011). The later board used the more modern WD2793 IC (MB8317) that combined the operation of these two ICs into one. Drawing numbers: MB1011 and 8317-2-06.
8328-2 Green mb8328_2_04_top mb8328_2_04_bottom
  • Used with the Series 3 Educator and Communicator and also with the PC85A.
  • This is the first Core board to support type 6264 8KB SRAMs and 8KB Mask ROMs.
  • Support is added for a rechargable RAM backup battery, including onboard automatic charging.
  • RAM config is either 16K (8x 2KB SRAMs or 2x 8KB SRAMs) or 32K (16x 2KB SRAMs or 4x 8KB SRAMs).
  • ROM config can be up to 72KB total capacity.
  • See Configuration Details section below for more information.
8328-4 Green 8328 4 Top
  • Used with the PC85B.
  • Supports the same RAM and ROM types as the 8328-2 Core board and adds support for type 27128 16KB EPROMs in the NET ROM and PAK1-5 ROM positions.
  • RAM config is either 16K (8x 2KB SRAMs or 2x 8KB SRAMs) or 32K (16x 2KB SRAMs or 4x 8KB SRAMs).
  • ROM config can be up to 120KB total capacity.
  • See Configuration Details section below for more information.
8328-6 Green 8328_6_04cr_top 8328_6_04cr_bottom
  • Used with the PC85B.
  • This board is almost identical to the 8328-4 Core board but has changes to the power-on reset circuitry around IC35.
  • The example pictured here is using 8KB 6264 RAM chips to give 32KB total RAM capacity.
  • See Configuration Details section below for more information.
8328-4 Premium, 8328-6 Premium
Green 8328_4_04cr_top
  • Used with the Premium PC85 and TeleTerm.
  • These are standard 8328-4 or 8328-6 Core boards that include 3 additional modifications required for Premium compatibility:
    • The Basic A ROM is upgraded to a 16KB 27128 EPROM. This provides a total of 24KB total capacity for Premium Basic.
    • The track between pin 26 and pin 28 of IC5 (Basic A ROM) is cut so that the ROM's A13 line is no longer tied to VCC.
    • A fly wire is added from pin 11 of X4 to pin 26 of IC5 (Basic A ROM) to provide the extra A13 address line required by the larger ROM.
  • The first and last of these 3 modifications can be seen in this 8328-4 example image. This board is also using 8KB 6264 RAM chips to give 32KB total capacity.
  • An earth strap for connecting to the Main board can also be seen at the lower left of this image.
  • The 50 way Expansion Interface is not fully functional on these modified boards. DMA functionality is compromised by the change in functionality of X4 pin 11 in Premium machines.
  • See Configuration Details section below for more information.

 

Configuration Details

 

Core board 8328-x

These are the final Static RAM Core boards to be produced. They were designed as a universal memory board to suit a range of Microbee models with differing capabilities and configurations. A variety of RAM and ROM IC types and configurations are supported and some non-intuitive programming techniques are required to control the various features of this board. The table below provides more specific detail on these boards and how to operate them.
 

IC

Function

Memory Map

Type 8328-2

Type 8328-4

Type 8328-6

  • Notes
IC1, IC2 and
IC6, IC11, IC16, IC7, IC12, IC17
RAM 0000h - 3FFFh 2KB 6116 SRAM or
8KB 6264 SRAM
as per 8328-2 as per 8328-2
  • If 8KB parts are used for IC1 & IC2 then these provide the whole 16KB capacity and IC6,11,16,7,12,17 are not required.
  • If 2KB parts are used for IC1 & IC2 then IC6,11,16,7,12,17 are populated with the same capacity parts to provide a total of 16KB.
  • IC21 (74HC138) must be fitted if 2KB parts are used, but is not required for 8KB parts.
IC3, IC4 and
IC8, IC13, IC18, IC9, IC14, IC19
RAM 4000h - 7FFFh 2KB 6116 SRAM or
8KB 6264 SRAM
as per 8328-2 as per 8328-2
  • If 8KB parts are used for IC3 & IC4 then these provide the whole 16KB capacity and IC8,13,18,9,14,19 are not required.
  • If 2KB parts are used for IC3 & IC4 then IC8,13,18,9,14,19 are populated with the same capacity parts to provide a total of 16KB.
  • IC21 (74HC138) must be fitted if 2KB parts are used, but is not required for 8KB parts.
IC5 Basic A ROM 8000h - 9FFFh 8KB Mask ROM or 8KB 2764 EPROM as per 8328-2 when used with Standard models. Also supports a 16KB 27128 EPROM (as 2x 8KB banks) when modified for the Premium PC85 as per 8328-4
  • The 16KB part used for the Premium PC85 requires bank switching.
  • In Machine Language, reset or set bit 5 of the "LV DAT" port (port 1Ch) to switch between the first and second banks of the 16KB ROM respectively, eg to select the first bank:
    • RES 5,A
    • OUT (1Ch),A
IC10 Basic B ROM A000h - BFFFh 8KB Mask ROM or 8KB 2764 EPROM or 8KB 6264 SRAM as per 8328-2 as per 8328-2
  • The TeleTerm uses an 8KB SRAM in this position rather than ROM.
IC15 PAK0/8 ROM C000h - DFFFh 8KB Mask ROM or 8KB 2764 EPROM as per 8328-2 as per 8328-2
  • In Basic, the PAK 0 or PAK 8 commands will both select this ROM.
  • In Machine Language, write 0 or 8 to port 0Ah, eg:
    • LD A,0
    • OUT (0Ah),A
IC20 NET ROM E000h - EFFFh 4KB 2732 EPROM or 8KB 2764 EPROM (as 2x 4KB banks) as per 8328-2 or 16KB 27128 EPROM (as 4x 4KB banks) as per 8328-4
  • The 8KB and 16KB parts, if used, require bank switching.
  • In Machine Language, load register A with the required 4KB bank number (0-3) and input from port 0Ah to switch. For example, to select bank 0:
    • LD A,0
    • IN A,(0Ah)
IC25 PAK1/9 ROM C000h - DFFFh 8KB 2764 EPROM as per 8328-2 or 16KB 27128 EPROM (as 2x 8KB banks) as per 8328-4
  • If the 8KB part is used then:
    • In Basic, the PAK 1 or PAK 9 commands will both select this ROM.
  • If the 16KB part is used then bank switching is required:
    • In Basic, the PAK 1 command selects the first bank of this ROM and PAK 9 selects the second bank.
  • In Machine Language, write the required PAK number to port 0Ah, eg:
    • LD A,1
    • OUT (0Ah),A
IC24 PAK2/10 ROM as per IC25 as per IC25 as per IC25 as per IC25 as per IC25, except different PAK numbers are used to select this IC.
IC23 PAK3/11 ROM as per IC25 as per IC25 as per IC25 as per IC25 as per IC25, except different PAK numbers are used to select this IC.
IC22 PAK4/12 ROM as per IC25 as per IC25 as per IC25 as per IC25 as per IC25, except different PAK numbers are used to select this IC.
IC27 PAK5/13 ROM as per IC25 as per IC25 as per IC25 as per IC25 as per IC25, except different PAK numbers are used to select this IC.

 

Dynamic RAM Core boards

Overview

  • Microbee RAM was changed from SRAM to DRAM in later models.
  • Banks of 8x 64Kb Dynamic RAM (DRAM) chips were used to provide either 64KB (1 bank) or 128KB (2 banks) total RAM capacity.
  • DRAM Core boards also included an integrated floppy disk controller (FDC).

 

Board Number Board Colour Component Side Image Solder Side Image
  • Notes
8312-2-08C
8312-3-11
Green 8312_2_08c_top 8312_2_08c_bottom
  • This was the first DRAM core board design. It was supplied with 64k or 128k of DRAM.
  • Used in the 64K CIAB. These images are of a 64K CIAB Core board.
  • One boot ROM was supplied. This was 8K but the design also allows for a 16K EPROM to be used in the same location. There are also 2 other ROM locations that were not used on production units.
  • The DRAM core board uses port 50H to manipulate the memory map that determines what ROMs are in memory and also controls the video location.
  • The board also featured an optional 4500A IC that could be used instead of the DRAM ICs, but it appears that this was never used on production models.
  • A location for an optional Z80 SCC and support parts was provided, but it appears that this also was never used on production models.
8342-2 Green 8342_2_04cr_top 8342_2_04cr_bottom
  • Similar functionality as the 8312 series but with a redesign of the physical layout.
  • The optional 4500A memory IC was removed.
  • Used as 64K Slave on Starnet.
8342-4 Green 8342_4_04cr_top 8342_4_04cr_bottom
  • Very similar design to 8342-2 but with some minor changes to component positioning and some components removed, such as RN1, R1, R9, R10 & D2. See ECN 17-130885 for more details on these changes.
  • Early 8342-4 Core boards are missing a track between pin 36 of IC35 and pin 6 of RN4, resulting in the disk controller chip (IC35) having no connection to the floppy drive write protect signal. The fault was corrected in boards produced after 26th November 1985 but there is no difference in board numbering between the two versions. See ECN 26-251185 for more details.
  • An extra earth strap/plug was fitted.

 

Premium Plus+ Core boards

Overview

  • Developed in 2012 by Microbee Technology especially for the limited edition Premium Plus+ release.
  • Uses modern surface mount components to provide RAM, ROM and FDC capability for the Premium Plus+.
  • Includes both physical and emulated FDC capability. Disk images for emulation are stored on SD card.
  • Also includes ethernet and fast serial ports, though these are not currently presented through to the Microbee (Z80) system.
  • Plugs into the PP+ Main board in the same way as the original Microbee Core boards but uses a different type of connector.

 

Board Number Board Colour Component Side Image Solder Side Image
  • Notes
21-01000-01 Green pp_top pp_bottom
  • Used in the Premium Plus+.
  • These images have the optional physical FDC fitted.
  • Provides a total of 1MB program RAM to the Z80 environment as 8x 128KB switchable banks.