Acorn Computers Limited Winchester disk 110, Руководство по обслуживанию

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In the following description, the Host Adapter is known as the "initiator",
and the disc controller is known as the "target".
5.1.1 SCSI control and data lines
The 8 control and 8 data lines on the SCSI side of the controller (shown on
the right side of the Host Adapter circuit diagram in the appendix) are all
active-low open collector, and are as follows:
SELECT (SEL, pin 44) is an open collector signal which is asserted by the
initiator as the first step in any transfer of data through the interface.
BUSY (BSY, pin 36) is an open collector signal which is asserted by the
target to indicate that the data bus is in use. This is the first response of
the target to the initiator's assertion of SEL, and the SEL/BSY handshake is
the first communication in any Winchester filing system operation.
CONTROL/DATA (C/D, pin 46) is asserted by the target when the bus carries
control information, and is deasserted when the bus carries data.
INPUT/OUTPUT (I/O, pin 50) controls the direction of data flow, and is
asserted by the target to indicate input to the initiator (disc to computer),
and is deasserted to indicate output to the target (computer to disc).
REQUEST (REQ, pin 48) is asserted by the target to indicate a request for a
REQ/ACK data transfer handshake.
ACKNOWLEDGE (ACK, pin 38) is asserted by the initiator to indicate
acknowledgement of a REQ/ACK data transfer handshake. The REQ/ACK handshake
provides the asynchronous timing of all data transfer between initiator and
target.
RESET (RST, pin 40) is asserted by the initiator on power-up and when the host
microcomputer's BREAK key is pressed. It causes the "reset condition" (see 5.
1.3) which immediately clears the bus and resets the system.
MESSAGE (MSG, pin 42) is asserted by the target when it issues a message byte
to notify completion of a command, see 5.1.2.
DATA BUS (DB0 to DB7, pins 2 4 6 8 10 12 14 and 16) is a parallel data bus
consisting of 8 signals from DBO (least significant) to DB7 (most significant)
. 1 byte of information is transferred across the bus with each REQ/ACK
handshake. It is important to remember that the data lines are active-low and
therefore are inverted in both directions when communicating with the host
microcomputer.
All odd numbered pins are OV, and pin 34 is +5V.
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