Yamaha Mio, Service Manual

Page: 59 / 73

Hardware Description
The MIO has 3 basic addressing areas: 1) ACIA at $D1C0-$D1DF, 2) MIO Latches at $D1E0-
$D1FF, and 3) 256 bytes of RAM at $D600-$D6FF. Note that the ACIA and MIO latches are not fully
decoded; both contain 4 read/write registers, but each register has 7 shadows. Tables 5-1 through 5-
3 briefly describes the MIO registers.
ACIA Operation
For more information on the ACIA operation, refer to a 6551A or 65C51 data manual (this part is
manufactured by Rockwell, GTE, NCR, and RCA). The only irregularity of usage is that the lines
DSR, CTS, and DCD are tied to ground. This is due to the fact that the ACIA will not receive data if
either of these lines are false. To read the actual state of these lines, you must read location $D1E3
(bits 2,1,0). This will return the true lines sense (DCD true is indicated by a high on bit 1 of $D1E3).
SASI/SCSI Interface
The data input/output ($D1E1) and input control lines ($D1E2) reflect the true voltage levels on the
ports. Thus, the input control lines are normally all ones (port voltages are +5) which represents a
logic false on the bus. For more information on SASI/SCSI bus protocol, refer to the Adaptec ACB-
4000 Series User's Manual, the XEBEC S1410A disk controller document, the Seagate ST225N
manual, or any other device manual employing these protocols.
The ACK-/REQ- handshake cycle is performed by the MIO hardware. Whenever the data ($D1E1) is
read or written, ACK- is set true. It is cleared by a high level on the REQ- input signal.
The RST- is set true when $D1E0 is read or when the RESET key pressed on the computer (or
during powerup). It is cleared by reading location $D1E2.
The I/O- controls whether the 8-bit printer/SASI registers are input or output. When I/O- is high (input
to SASI controller), the data is output on the printer and SASI port. If I/O- is low, then the output
latches are disabled (it will latch new data, but the output is tri-stated).
Printer Interface
The printer data ($D1E1) and BUSY signals are high true logic. The a high (1) on BUSY (bit 6 of
$D1E2) indicates the printer is busy. A low (0) on FAULT- (bit 4 of $D1E2) reflects an error condition
in the printer (printer off?).
MIO Service Manual Page 58