The device outer enclosure was tamper evident but the device itself was tamper proof HSM, basically. Any kind of intrusion (melting, dissolving, drilling, etc.) into a secure internal enclosure (separate processor, memory and battery) would cause internal battery to be disconnected from internal SRAM and basically the device would loose all cryptographic material and then self-destruct.
To give a bit of background, when you type your PIN on credit card terminal it is not the terminal application that is really getting the pin (well, except for special credit cards but that is really problem of the Bank that issued the card). The Visa/Mastercard mandate that the application don't have control over the PIN and that the PIN entry uses physically separate keyboard and display.
To achieve this, the keyboard and the display is galvanically separated for the duration of the PIN entry and the PIN is transferred directly to the HSM where it is being encrypted before it is transferred to the application processor for the rest of processing.
To give a bit of background, when you type your PIN on credit card terminal it is not the terminal application that is really getting the pin (well, except for special credit cards but that is really problem of the Bank that issued the card). The Visa/Mastercard mandate that the application don't have control over the PIN and that the PIN entry uses physically separate keyboard and display.
To achieve this, the keyboard and the display is galvanically separated for the duration of the PIN entry and the PIN is transferred directly to the HSM where it is being encrypted before it is transferred to the application processor for the rest of processing.