Service manual

Chapter 4 E40S/E60S Circuit description
FileStore Service Manual 96
The information transfer phase is as follows:
Signals Direction of information
Msg C/D I/O Phase name 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
0 0 0 message in phase target to initiator
All signals active-low: 0 = assertion
1= deassertion
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.
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.
Each information transfer phase is now described in turn:
 Command phase
Allows the initiator to direct the subsequent action of the target by transferring command
bytes. The target asserts C/D and deasserts MSG and I/O.
 Status phase
Allows the initiator to read the target‘s status information. The target asserts C/D and I/O
and deasserts MSG.
 Data out phase
Allows data to be transferred from initiator to target. The target deasserts MSG, C/D and
I/O.
 Data in phase
Allows data to be transferred from target to initiator. The target asserts I/O and deasserts
MSG and C/D.
 Message out phase
Not used by the system - available for future expansion.
 Message in phase
Allows the target to send a message byte to notify completion of a command.