CASSETTE/KEYBOARD INTERFACE and KEYBOARD (parts)
The Cassette/Keyboard Interface contains logic for:

- the TRS-80 cassette input/output,
- the BASICODE cassette input/output,
- the cassette motor control for both cassete recorders,
- 1 watt sound amplifer,
- reserved space for optional keyboard interrupt applications,
- drivers and buffers for keyboard and keyboard interface,
- address decoding for keyboard and cassette,
- 32/64 character mode of the Video screen,
- BUS-termination.

The Keyboard contains logic for:
- alpha-lock,
- numeric keyboard.

The inputs and outputs for cassette operation in TRS-80 and BASICODE are realized with two DIN-connectors. The board supports two cassette recorders for reading and writing tapes. The motors of both recorders can be switched under software control. The standard issue of the computer has components for one cassette recorder. On request (and additional costs) a board supporting two recorders is available.

Switching the cassette recorder motor signal is done with bit 2 of port 255. Setting this bit to 1 starts the recorder.
If two recorders are used, bit 0 of memory location 37E4h switches between the first (bit 0 = 0) and the second (bit 0 = 1) recorder.
Recorder 1 is connected to connector P3, next to the keyboard header. Recorder 2 is connected to connector P4.
The ICs and logic creating these functions are: 
SW1, SW2, Z3, Z4 and Z5.

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In TRS-80 mode the keyboard is read by the polling principle. This requires the keyboard to be scanned every time to check a key is pressed.
The design of the Aster CT-80 allowed the option of an "interrupted" keyboard mode. This works with the Z80 processor "Interrupt mode 2". This requires an Interrupt Vector is stored in the keyboard logic. This vector is presented to the processor on request (Interrupt Acknowledge). The vector is set by writing to memory in the keyboard range (3800h-38FFh). Bit 3 of port FFh controls the keyboard interrupt.
The ICs for this functionality (Z15, Z16 and Z17) are not present on the defalt issue of the board.

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The keyboard board
The numeric part of the keyboard behave different from the TRS-80 numeric part in that they do not respond differently if SHIFT is pressed. This is realised by the logic in ICs Z6, Z57, Z8 and Z9. 
This function is made possible by using the scann routines if the BASIC ROMs or the scan routine from CP/M. Software which do not scan the complete keyboard like games may not work properly with this feature. The numeric keyboard should not be used by these games. When a numeric key is pressed in this situation, the main keyboard can get disabled. The main keyboard can be resored by pressing the right-RESET key. This resets only the keyboard, not the computer.

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The keyboard also has an 'ALPH-LOCK' key. This key has a mechanical toggle. A LED will light to indicate the Lock situation. All alpha-keys are now seen as capitals. This function is generated by a circuit including Z1, Z2, Z3, Z4, Z5, Z6 and Z10. Part of the logic detects a alpha-key is pressed and simulates a pressed SHIFT key during the scan.