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Microbee Core boards

Static RAM Core boards

Overview

  • Early versions of the Static RAM Core 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 added support for 8KB SRAM chips.
  • SRAM Core boards can be fitted with a battery (4-5 volts) that allows the RAM contents to be preserved while the machine is powered off. Early versions used a non-rechargable battery for this, but later versions changed to a rechargable NiCd battery.
  • Starting with support for 4KB EPROMs, later versions added support for 8KB and 16KB EPROMs as well as 8KB Mask ROMs.
  • All SRAM Core boards have provision for a 50 way Expansion Interface at the rear, though most do not have the associated connector fitted as standard.
Board Number Component Side Image Solder Side Image
  • Notes
1632 REV D
1632 REV D Component Side (Blue)
1632 REV D Component Side (Blue)
1632 REV D Component Side (Green)
1632 REV D Component Side (Green)
1632 REV D Solder Side (Blue)
1632 REV D Solder Side (Blue)
1632 REV D Solder Side (Green)
1632 REV D Solder Side (Green)
  • Used with 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.
  • PAK/EDASM/Wordbee option config is 2x 2532 4KB EPROMs.
  • Net option config is 1x 2532 4KB EPROM.
  • Exists in both Blue and Green coloured variants.
MB1732
MB1732 Component Side
MB1732 Component Side
MB1732 Solder Side
MB1732 Solder Side
  • 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.
  • These images show a number of modifications (ie, fly wires, solder bridges etc). These are believed to be factory fitted modifications required to correct board faults.
MB1732-33
MB1732-33 Component Side
MB1732-33 Component Side
MB1732-33 Solder Side
MB1732-33 Solder Side
  • 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.
  • Earlier MB1732 board faults have been fixed with this board and provision for an LED in the lower right corner has been removed.
  • "(C) Copyright 1983 by MICROWORLD" is now screen printed in the top right corner.
MB1656
MB1656 Component Side
MB1656 Component Side
MB1656 Solder Side
MB1656 Solder Side
  • 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.
  • The 50 way expansion port connector was normally factory fitted to this board to provide connectivity to an external box that contained a floppy disk controller and floppy drives. Alternatively, the boot ROM's monitor program could be used to load Basic/EDASM from tape.
  • There were 2 versions of the floppy controller used in the external box. The early version used WD1793 and WD1691 controller ICs (MB1011). The later version used a more modern WD2793 IC (MB8317) which combined the operation of the earlier two ICs into one. Drawing numbers: MB1011 and 8317-2-06.
MB8328-2
MB8328-2 Component Side
MB8328-2 Component Side
MB8328-2 Solder Side
MB8328-2 Solder Side
  • 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.
MB8328-4
MB8328-4 Component Side
MB8328-4 Component Side
MB8328-4 Solder Side
MB8328-4 Solder Side
  • Used with the PC85B.
  • Supports the same RAM and ROM types as the MB8328-2 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.
MB8328-6
MB8328-6 Component Side
MB8328-6 Component Side
MB8328-6 Solder Side
MB8328-6 Solder Side
  • Used with the PC85B.
  • This board is almost identical to the MB8328-4 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.
MB8328-4 Premium
MB8328-6 Premium
MB8328-4 Premium Component Side
MB8328-4 Premium Component Side
MB8328-4 Premium Solder Side
MB8328-4 Premium Solder Side
  • Used with the Premium PC85 and TeleTerm.
  • These are standard MB8328-4 or MB8328-6 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 MB8328-4 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 MB8328-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

MB8328-2

MB8328-4

MB8328-6

  • Notes
IC1, IC2 and
IC6, IC11, IC16, IC7, IC12, IC17
RAM 0000h - 3FFFh 2KB 6116 SRAM or
8KB 6264 SRAM
as per MB8328-2 as per MB8328-2
  • 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 MB8328-2 as per MB8328-2
  • 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 MB8328-2 when used with Standard models. Also supports a 16KB 27128 EPROM (as 2x 8KB banks) when modified for the Premium PC85 as per MB8328-4
  • 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 MB8328-2 as per MB8328-2
  • 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 MB8328-2 as per MB8328-2
  • 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 MB8328-2 or 16KB 27128 EPROM (as 4x 4KB banks) as per MB8328-4
  • 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 MB8328-2 or 16KB 27128 EPROM (as 2x 8KB banks) as per MB8328-4
  • 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).
  • The DRAM core boards use port 50H to manipulate the memory map. This determines whether the ROM is mapped into memory and also controls the video location.
Board Number Component Side Image Solder Side Image
  • Notes
MB-168
8312-13
8312-13 Component Side
8312-13 Component Side
8312-13 Solder Side
8312-13 Solder Side
  • The MB-168/8312-x boards were the first DRAM core board design. These boards were supplied with either 64k or 128k of DRAM fitted.
  • 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 board has provision for a TMS 4500A Dynamic RAM Controller IC, but it appears that this was never used on production models.
  • This board also has provision for a Zilog SCC IC to provide a serial communications port via a 15 way D connector at the rear, but it appears that this was never fitted to production models either.
  • While the numbering of this board would seem to indicate that it chronologically follows board 8312-2, a number of the modifications that can be seen in these images appear to be implementing connectivity that aligns with the 8312-2 schematic. For this reason it is assumed to pre-date 8312-2 and that the modifications are board fault corrections.
  • The example in these images appears to have had a number of components removed (ie RAM, FDD connector and potentially RV2) and shows some odd reshaping of the heatsink and PCB cutouts.
MB-168
8312-2
8312-2 Component Side - CIAB
8312-2 Component Side - CIAB
8312-2 Component Side - Star Network Workstation
8312-2 Component Side - Star Network Workstation
8312-2 Solder Side - CIAB
8312-2 Solder Side - CIAB
8312-2 Solder Side - Star Network Workstation
8312-2 Solder Side - Star Network Workstation
  • The first pair of images are from a 64K CIAB.
  • The second pair are from a Star Network Workstation. Note that the disk controller components are not fitted in this case.
MB8342-2
MB8342-2 Component Side
MB8342-2 Component Side
MB8342-2 Solder Side
MB8342-2 Solder Side
  • The MB8342-x boards superceded the earlier MB-168/8312-x boards. They provide similar functionality but have a redesigned physical layout.
  • Provision for the 4500A Dynamic RAM Controller is removed in this model, but the Zilog SCC is retained and a second 15 way D connector is added at the rear for a second serial port. Again, it appears that the SCC and associated serial port capability was never implemented on production models.
  • The disk controller components have not been fitted to the board in these images indicating that it was most likely used in a Star Network Workstation.
MB8342-3
MB8342-3 Component Side
MB8342-3 Component Side
MB8342-3 Component Side with 40 way
MB8342-3 Component Side with 40 way
MB8342-3 Solder Side
MB8342-3 Solder Side
  • One of these images shows a board that does not have any FDD controller components installed and has a 40 way drive connector fitted. This implies that it was used with a HDD, however a BN54 ROM is installed which does not support HDDs. It is believed that this is a Star Network Workstation Core board and that the 40 way connector was never intended to be used. The connector was apparently factory installed on some Star Network Workstation Core boards just to minimise the number of board variations produced. The same board could then also be used for HDD models by simply installing the appropriate ROM.
MB8342-4
MB8342-4 Component Side
MB8342-4 Component Side
MB8342-4 Solder Side
MB8342-4 Solder Side
  • 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 earth strap/plug is 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+.
Board Number Component Side Image Solder Side Image
  • Notes
21-01000-01
21-01000-01 Component Side
21-01000-01 Component Side
21-01000-01 Solder Side
21-01000-01 Solder Side
  • Used in the Premium Plus+.
  • These images have the optional physical FDC components fitted.
  • Provides a total of 1MB program RAM to the Z80 environment as 8x 128KB switchable banks.
  • Provides both physical and emulated FDC capability. Disk images for emulation are stored on SD card.
  • Also provides 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 adds a 3rd 2 way connector that can be seen adjacent to the SD card holder in the solder side image.