SCSI Signals
In addition to the data bus and parity signals, a parallel SCSI bus contains nine control signals:
| Signal name | Meaning when asserted (deasserted) |
|---|---|
| BSY Busy | Bus in use (bus free) |
| SEL Select | Asserted by the winner of an arbitration, during selection by an initiator or reselection by a target |
| RST Reset | Initiator forces all targets and any other initiators to do a warm reset |
| C/D Control/Data | * Bus contains control information (bus contains data) |
| I/O Input/Output | * Transfer is from target to initiator (transfer is from initiator to target). Also asserted by a target after winning arbitration to indicate reselection of an initiator. |
| MSG Message | * Bus contains a message (bus contains data or command/status) |
| REQ Request | Target requests initiator to transfer the next unit of information on the bus, as indicated by the 3 phase signals (no request) |
| ACK Acknowledge | Initiator acknowledges target request, completing the information transfer handshake (no handshake) |
| ATN Attention | Asserted by an initiator after winning arbitration to select a target. |
Notes: * One of 3 signals which are driven by a target during information transfer to indicate the Bus Phase
There are also three DC levels:
| Signal name | Usage |
|---|---|
| TERMPOWER | See the Termination section for details |
| DIFFSNS | Grounded in single-ended buses, otherwise floats to a positive voltage |
| GROUND | Most spare pins in the connector are designated as grounds |
There are three electrically different variants of the SCSI parallel bus: single-ended (SE), high-voltage differential (HVD), and low-voltage differential (LVD). The HVD and LVD versions use differential signaling and so they require a pair of wires for each signal. So the number of signals required to implement a SCSI bus is a function of the bus width and voltage:
| Bus Width | Voltage | Data | Parity | Control | TERMPOWER | DIFFSNS | GROUND | Reserved | Total |
|---|---|---|---|---|---|---|---|---|---|
| 8-bit | SE | 8 | 1 | 9 | 1 | 1 | 30 | 0 | 50 |
| 8-bit | LVD/HVD | 16 | 2 | 18 | 1 | 1 | 12 | 0 | 50 |
| 16-bit | SE | 16 | 2 | 9 | 4 | 1 | 34 | 2 | 68 |
| 16-bit | LVD/HVD | 32 | 4 | 18 | 4 | 1 | 7 | 2 | 68 |
Read more about this topic: Parallel SCSI
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