Microbee Add-on boards

Additional functionality can be added to various Microbee models though optional add-on boards.
 

Standard Colour option

This board is an option for Standard Microbees with a 3.375MHz CPU clock. It adds a new colour video output via a 15 way D connector that exits the case directly beneath the standard 25 way Serial port connector. RGB video output is digital (TTL level) and/or analog depending on the particular colour board fitted and is not CGA compatible. The board is installed beneath the Main board and interfaces with it via a complicated array of wiring, plugs, sockets, headers and Main board modifications.

The colour board adds a second byte of RAM for each screen character, and allows each character to have 2 colours. The foreground colour has 32 possible values (5 bits) that are determined by the contents of the 82S123 PROM located on the colour board. Not all foreground colours combinations are unique with only 27 being possible. The background colours were made up from the remaining 3 bits of the colour RAM byte (8 colours) and another 3 bits on a colour control port that control the RGB background intensity level. This allowed the intensity of each of the RGB colours to be set to full or half intensity but only 7 different values could be in effect at any one time and changing the intensity value would affect the entire display. The colour pallette provided by these boards differs from that provided by the later Premium Series Microbees.

In addition to colour functionality, this board also provides a video deglitch function, removing the unsightly video glitches that normally occur on Standard Microbees during periods of CPU access to the video RAM. This is achieved by forcing CPU activity to wait for times when the video RAM is not being used for image display and so does have some effect on performance.


 

Viatel upgrade option

This board is an option for Standard Microbees with a 3.375MHz CPU clock. It adds the ability to generate a 40 character wide display that occupies the full screen width. Without this board fitted, the 40 column Viatel/Videotex display will only occupy half of the available screen width (ie, the display will be 80 columns wide instead, with 40 active Viatel/Vedeotex columns displayed in the centre and 20 column blank areas on either side). Premium Series Microbees have the same full width 40 column functionality built-in as standard, so this add-on board is not required.

Technically, the board works by halving the frequency of video clock when operating in 40 column mode. Switching between normal and 40 column mode is under software control and is achieved by reading from port 09. For example, to switch into 40 column mode:

• LD A,01h
• IN A,(09h)

and to switch back to normal mode:

• LD A,00h
• IN A,(09h)

Details of this board, together with installation instructions can be found in Section F of the Microbee Hardware Notebook and also in the July 1985 edition of Australian Electronics Monthly (AEM) magazine.


 

50 Way Expansion Kit

This is an add-on board designed for Standard models. It provides a 50 way Expansion Interface for connecting external peripherals and also provides video deglitch functionality. The two functions are not related to each other and may be independently and optionally implemented during installation. The 50 way interface is identical to that normally provided by Static RAM (SRAM) Core boards and may be used to provide a second interface for those machines. The board is particularly useful for adding expansion capability to models that are fitted with Dynamic RAM (DRAM) Core boards since these do not incorporate a 50 way interface.

This board is installed beneath the Standard Main board and connects via an extended pin/socket arrangement as well as a set of separate fly wires. See the 50 Way Expansion Kit Manual for construction and fitting details.


 

Experimenter board

The Experimenter board connects to the Microbee's 50 way Expansion port and provides buffered access to the Z80 bus for hardware experiments, custom additions and protoyping. The board was supplied as a kit, which included the PCB and all components necessary to provide the interface buffering and connectivity to the Microbee. Also included is a Z80A PIO and 15/25 way D connectors which could be used to provide additional Parallel/Serial port capability or other customised input/output functionality. The board provides its own power supply and has a large on-board prototyping area for user experiments and additions.

More information can be found in the Experimenter board manual.


 

64K ROM Pack

This board is designed for ROM based Microbees and connects to the 50 way expansion port. It can accommodate up to 8x 8KB EPROMS (type 2764) to provide a total of 64KB additional ROM capacity. The board can also be modified to support 4KB EPROMS (type 2732) if required. The board is powered from the Microbee via the 50 way connector and can be accommodated inside the Microbee case underneath the Main board, or it can remain external.

Only one of the installed EPROMs can be selected at a time and it will be mapped to memory location C000h-DFFFh. Any EPROMs internal to the Microbee that already map to that location will conflict and are normally removed and installed into the ROM Pack instead. Alternatively, the ROM Pack manual describes some hardware modifications that can be made to allow the internal EPROMs to remain in place and be automatically disabled whenever the ROM Pack board is attached.

The Basic "PAK n" command can be used to select the specific ROM Pack EPROM required where "n" is a number from 0-7. This command simply writes the PAK number to Z80 port 0Ah which the ROM Pack board then decodes to select the appropriate EPROM. The same selection operation can therefore be performed by the Basic command "OUT 10,n" or the Z80 OUT instruction.

The board has provision for a battery to be installed which appears to allow the last PAK selection to be retained between power cycles, however this feature is not mentioned in the manual.


 

Real Time Clock option

This board adds a battery backed Real Time Clock (RTC) facility to early Standard and Premium model Microbees. It is installed beneath the Main board and connects via a set of 16 fly wires.

The final version of the Standard Main board (type 1248-6) incorporates built-in RTC functionality so this additional add-on board is therefore not required for that version. It is suitable for use with all other Standard Main board versions. The 1248-6 Main board RTC circuit and functionality appears to be identical to that provided by the add-on board.

The first version of the Premium Main board (type 8501-2) suffered from board errors that made it difficult use the built-in RTC facility so this add-on board was recommended for use with that version instead. Later Premium Main board versions (types 8501-3 and 8501-4) corrected these errors and this add-on board is no longer required.

More information can be found in the Microbee Hardware Notebook, chapters 8 and 10.


 

Screen Deglitch board

This board provides video deglitch functionality for Standard Microbees. The final version of the Standard Main board (type 1248-6) incorporates built-in video deglitch functionality so this additional add-on board is therefore not required for that version. It is suitable for use with all other Standard Main board versions.

The deglitch circuitry and functionality provided by this small add-on board is effectively the same as that provided by both the 1248-6 Main board and the 50 Way Expansion Kit. Video deglitch is achieved by forcing CPU activity to wait for times when the video RAM is not being used for image display and so does have some effect on performance.

The Microbee Hardware Notebook indicates that this board was fitted mainly to Starnet student stations. More information can be found in Section F of that document, chapter 7.