Short as usual:

The camera is a little scratched by claws from either pussycats, bats, vampires, hedgehogs, lizards and vacuum tubes but the picture is clear enough for some explanations.

The system acts like the 'man in the middle' concept. It is connected between the tape drive and mainframe computer - one i/o buffer for each one.

The STM32 microcontroller is also connected by another bidirectional buffer which also acts as a level shifter.  Datasheet for STM32F429 says the GPIOs are 5V tolerant but I do not want to risk.  There is a microSD card connected to the STM32 but not shown in the schematic above.

Each buffer can be activated or switched to high impedance state - meaning the corresponding equipment is isolated.

Commands are selected from the user panel, information is displayed on the VFD and additional debugging service-mode data is available through a serialUSB line.

The following functions are supported:

1. Host mode:    tape drive buffer is activated, mainframe buffer is isolated, the STM32 level shifter buffer has write+control lines as outputs and read+status lines as inputs;

2. Drive mode:  tape drive buffer is switched to high impedance state as we don't need to talk to the drive. Mainframe buffer is activated and the STM32-buffer has write+control lines as inputs (accepting data) and read+status lines as outputs (transmitting data);

3. Bypass mode:   tape drive and mainframe buffers are both activated and they can talk to each other while the logic analyzer is eavesdropping on the love story flowing through the data bus. The STM32 level-shifter buffer separates the microcontroller from the rest by switching to high-impedance mode.

4. Monitor/service mode:   all debugging stuff is activated and slowing down the system. Either bypass, host or drive modes are monitored to help debug i/o glitches or software bugs.

Everything fits inside a 4-U rack mounted box just like this:

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