Atari 1040STE, Field Service Manual

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controller. The system is interfaced to the outside via two inverters on the transmit
side and an LED/photo–transistor chip on the input side. The input signal is also
routed around through two inverters to the output connector where it is called MIDI
THRU in order to allow chaining of multiple devices on the MIDI bus.
MIDI Out
1 – THRU Transmit Data
2 – Shield Ground,
3 – THRU Loop Return 3
4 – OUT Transmit Data
5 – OUT Loop Return
MIDI In
1 – Not Connected
2 – Not Connected 3
3 – Not Connected
4 – IN Receive Data
5 – IN Loop Return z
Figure 2–2 . MIDI Ports
2.12.2 Intelligent Keyboard
The keyboard transmits make/break key scan codes, ASCII codes, mouse data, and
joystick data, in response to external events, and time–of–day (year, month, day,
hour, minute, second) in response to requests by the CPU. Communication is
controlled on the main board by a 6850 device and on the keyboard assembly by the
1 MHz 8 bit HD6301 Microcomputer Unit. The HD6301 has internal RAM and ROM.
Included in ROM are selftest diagnostics which are performed at power–up and
whenever the RESET command is sent over the serial communication line by the
CPU. The MC6850 is read from and written to by the CPU in response to interrupts
which are passed to the CPU by the MFP interrupt controller.
2.12.3 Mouse/Joysticks
The 2 Button Mouse is an opto–mechanical device with the following characteristics:
a resolution of 100 countslinch, a maximum velocity of 10 inches/second and a
maximum pulse phase error of 50 percent. The joystick/mouse port has inputs for up,