3.2 Memory Mapped I/O In order to easily control the vast number of hardware functions that the 8031 needs to access, a system of memory mapped I/O is used. The basic idea is to make hardware functions appear to the 8031 as unused memory locations. That way all that the software has to do is write to a memory location in order to send that information to a specific device such as the LCD, or ASIC. 74HC138 (U13) performs the majority of the work in this circuit. Two things are required before U13 becomes active. 1> A15 must be low (i.e. the 8031 is accessing the lower 32K of address space). 2> The 8031 WRite line must be active (the 8031 is performing a memory write).
A15 is used to directly control which function (memory or I/O) is active. Once U13 is enabled, addresses A8-A10 are decoded by it, and the latch corresponding to the value of the decoded address is strobed. At this point, data on the 8031 data buss is "written" into the latch. 3.3 Keypad I/O Keypad I/O is handled through a simple polling process Each row of the keypad matrix is pulled low one at a time (via U14 which is memory mapped). If any button along the row is pressed, the corresponding column input (U22) will appear high. If no buttons are pressed, all column inputs will appear as a low. D9- D15 and R42-R47 provide protection for the outputs of U14.
Use diagram 3 to localize individual button failures. 3.4 MIDI I/O The MIDI hardware is a standard implementation. MIDI out begins at the 8031's TXD port (pin 11) and travels via R6 to the darlington pair Q1/Q2. Note that the 8031's internal pullup is not very strong, and older units (revision A) may require the addition of an external pullup resistor for the MIDI out to function correctly (see section 7.7). MIDI in consists mostly of the opto isolator (U4), protection diode D6, pullup R7, and threshold resistor R5. Note that the threshold resistor may need to be changed in order to eliminate false MIDI triggers (see section 7.6). Diagram 3 HR-16/HR16B Service Manual 1.00 4