Beta Disk 128C:
INTRODUCTION:

Beta Disk 128C is a controller for 1 to 4 floppy disk drives for ZX Spectrum and compatible computers. It comes from the original interface of the same name from British company named Technology Research Ltd, with which it is fully compatible. The design is virtually unchanged from the original wiring, with just a few changes, the description of which will go further. This ensures compatibility with the original interface. "128" in the interface name indicates that it is a version of the ZX Spectrum operating with both 48KB and 128KB RAM. "C"  indicates Czech clone :-), or controller producer CSS Electronics, as you like ;-)


PARAMETERS:

• Optional to connect 1 to 4 floppy drives 3.5" or 5.25"
• Fully compatible with the ZX Spectrum 48KB, 128KB "toast rack" and 128K +2 Amstrad gray box. It is also compatible with other ZX Spectrum clones with a standard system connector and ROM compatible with ZX Spectrum 48KB or 128KB, for example Didaktik M. Didaktik Gama can be connected after additional fixing, see next description. Interface is incompatible with the ZX Spectrum +2A, +2B and +3, these have non-standard signals ROMCS (2x) on the edge connector and incompatible ROM.
• The device is realized on two printed circuit boards horizontally connected by connectors
• Fully through bus on the bottom board, drivers for all signals for the controller and memory on the top board
• Used original FDC1793 floppy disk controller or analog
• Integrated voltage converter from +5V to +12V for controller 1793
• Possibility to use 16KB EPROM for TR-DOS system (as original), or 32KB EPROM for two different versions of the system TR-DOS, system selection by jumper/external switch
• LED power-on indication (green LED) and TR-DOS system activation (red LED)
• The bottom board (buses and drivers) are powered directly from the computer, top board (TR-DOS and floppy controller) is powered by external source 7,5-9V/300mA max. via the rounded DC power source connector 3,5/1,4mm (positive pole center)


COMPATIBILITY:

Because the schematic is largely taken from the original interface, is directly compatible with these computers without any further modifications:
• ZX Spectrum 48KB
• ZX Spectrum 128KB "toast rack" with radiator to the right
• ZX Spectrum 128KB +2 Amstrad gray box
• Didaktik M
Compatibility depends on whether your computer is equipped with a system connector with a standard distribution of the appropriate signals, which the Beta Disk 128C interface uses.

ZX Spectrum 128KB +2A, +2B and +3: This computers is incompatible in terms of the ROMCS signal (25B), which is doubled with them. In addition, they also contain incompatible BASIC ROM. Therefore, the Beta Disk 128C interface can not be used with them!

Didaktik Gama 80KB, 192KB: Didaktik Gama computers of all versions meets the compatibility of the system connector, but they have an internal 8255A parallel interface using the same port addresses (31, 63, 95, 127), as Beta Disk controller, you can not use this computer without any modifications with the Beta Disk! Otherwise, there will be a collision of the 8255A parallel interface with the controller 1793 on the bus, and they could be damaged!


LIST OF CHANGES FROM ORIGINAL INTERFACE:

In electrical wiring, these changes were made to the original original interface of Technology Research Ltd:
• Drivers to amplify to ALL signals entering the interface from the computer: A0-A15, D0-D7, /RD, /WR, /IORQ, /MREQ, /M1, /RESET
• The controller and TR-DOS board have an external power supply due to less power to the computer's power source
• All diodes in logic strings are replaced by Schottky diodes - improved switching characteristics and lower voltage drop
• Most pull-up resistors are unified to 5K6 - ideal for LS-TTL logic + adding pull-up resistors to multiple locations, where they should have been, but were not originally
• Optimizing the use of TTL gates, instead of the original 74LS04 newly 74LS14 with Schmitt input for better separation of the signal from the noise
• Original LS-TTL+quarz oscillator replaced by prepared SMD oscillator 4.000MHz
• The board is an integrated regulator +5V and converter from +5V to +12V for floppy controller 1793 (voltage +12V is disconnectable by a jumper JP2)
• It is possible to mount 16KB EPROM 27128, or 32KB EPROM 27256, for 32KB EPROM the choice of half is done by jumper JP3 or an external switch connected to this jumper
• LED power supply indication + LED active TR-DOS indication
• GMBLK output signal added (can be interrupted by a jumper JP1) to disconnect internal peripherals on a computer with the same port addresses, what Beta Disk is using (31, 63, 95, 127, 255). This will prevent a collision between the Beta Disk and the internal periphery on your computer

Beta Disk 128C
Front edge connector
Rear edge and floppy connectors
Left side power DC connector
Both PCB
Both PCB
Both PCB