Acorn Computers Limited Winchester disk 110, Service Manual, Page: 16 / 43

The phases are as follows:
Bus free phase: indicates that the bus is available for use. The bus free
phase is indicated by all control signals described in section 5.1.1 being
deasserted. If SEL and BSY and RST are not asserted, that is sufficient to
guarantee bus free.
Selection phase: allows the initiator to select the target. After detecting
bus free, the initiator asserts SEL. The target detects SEL asserted, and BSY
and I/O deasserted, and responds by asserting BSY. The initiator deasserts
SEL and may then change the data signals.
Information transfer phases: allow transfer of information across the bus.
There are several different types of information transfer phase, and the type
is determined by MSG, C/D and I/O. Table 1 shows the information transfer
phases:
SIGNALS DIRECTION OF
MSG C/D I/O PHASE NAME INFORMATION TRANSFER
1 1 1 data out phase initiator to target
1 1 0 data in phase target to initiator
1 0 1 command phase initiator to target
1 0 0 status phase target to initiator
0 0 1 message out phase initiator to target (not used)
0 0 0 message in phase target to initiator
All signals active-low: 0=assertion
1=deassertion
Table 1 Information transfer phases
The information transfer phases use the REQ/ACK handshake to control
information transfer: each REQ/ACK allows the transfer of 1 byte. The
handshake sequence is:
1- target asserts REQ to request data transfer
2- initiator asserts ACK when data is valid on bus
3- target deasserts REQ when data has been transferred
4- initiator deasserts ACK ready for next handshake
Prior to and during information transfer, the I/O signal determines the
direction of the transfer as can be seen in Table 1.
Before each information transfer phase the target will set up the MSG, C/D and
I/O lines in such a way that these control signals are stable for 450ns before
the REQ of the first handshake, and remain valid until the deassertion of ACK
at the end of the last handshake.
During each information transfer phase the BSY line remains asserted and SEL
deasserted.
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