SCSI

Anatomy of a SCSI Bus

SCSI Is a bus based system, all devices on the bus have a unique ID and are split into two categories, Initiator, and Target. In general Initiators are show as an adapter connected to a host, and Targets are shown as controllers attaches to a target device. This abstraction serves to represent that there can be multiple possible targets behind a single controller, which share a single bus connection.

As SCSI is not a purely point-to-point bus, and allows for multiple bus initiators, there are three possible bus topologies. The first is Single Initiator, Single Target. This topology acts as a point-to-point bus between a SCSI Initiator and SCSI Target. The second being Single Initiator, Multiple Target. This topology is the traditional topology along with Single Initiator that people think of when they think of SCSI. The final possible common bus topology is Multiple Initiator, Multiple Target. This is where there are multiple initiators and multiple targets all on the shared bus.

graph scsi_bus_sist { fontname="Fira Code" bgcolor="#FFFFFF00" node [fontname="Fira Code"] edge [fontname="Fire Code"] rankdir=LR; node [shape = Mrecord]; H0 [label = "{<f0>HOST|<f1>ADAPTER}"]; T0 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; H0:f1 -- T0:f0 }

Single Initiator, Single Target Bus Topology

graph scsi_bus_simt { fontname="Fira Code" bgcolor="#FFFFFF00" node [fontname="Fira Code"] edge [fontname="Fire Code"] rankdir=LR; node [shape = Mrecord]; H0 [label = "{<f0>HOST|<f1>ADAPTER}"]; T0 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; T1 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; H0:f1 -- T0:f0 H0:f1 -- T1:f0 }

Single Initiator, Multiple Target Bus Topology

graph scsi_bus_mimt { fontname="Fira Code" bgcolor="#FFFFFF00" node [fontname="Fira Code"] edge [fontname="Fire Code"] rankdir=LR; node [shape = Mrecord]; H0 [label = "{<f0>HOST|<f1>ADAPTER}"]; H1 [label = "{<f0>HOST|<f1>ADAPTER}"]; H2 [label = "{<f0>HOST|<f1>ADAPTER}"]; T0 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; T1 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; T2 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; T3 [label = "{<f0>CONTROLLER|{<f1>TARGET|{LUN0|...}}}"]; H0:f1 -- T0:f0 H0:f1 -- T1:f0 H0:f1 -- T2:f0 H0:f1 -- T3:f0 H1:f1 -- T0:f0 H1:f1 -- T1:f0 H1:f1 -- T2:f0 H1:f1 -- T3:f0 H2:f1 -- T0:f0 H2:f1 -- T1:f0 H2:f1 -- T2:f0 H2:f1 -- T3:f0 }

Multiple Initiator, Multiple Target Bus Topology

Todo

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digraph scsi_with_arbitration{ fontname="Fira Code" bgcolor="#FFFFFF00" node [fontname="Fira Code"] edge [fontname="Fire Code"] rankdir=LR; node [shape = none]; _ node [shape = doublecircle]; BF node [shape = circle]; _ [label = "RESET"]; BF [label = "BUS FREE"]; A [label = "ARBITRATION"]; S [label = "SELECTION"]; R [label = "RESELECTION"]; C [label = "COMMAND"]; D [label = "DATA"]; ST [label = "STATUS"]; M [label = "MESSAGE"]; subgraph CDSM { C -> C C -> D C -> ST C -> M D -> D D -> C D -> ST D -> M ST -> ST ST -> C ST -> D ST -> M M -> M M -> C M -> D M -> ST } S -> C [label = ""]; S -> D [label = ""]; S -> ST [label = ""]; S -> M [label = ""]; R -> C [label = ""]; R -> D [label = ""]; R -> ST [label = ""]; R -> M [label = ""]; _ -> BF [label = ""]; BF -> A [label = ""]; A -> S [label = ""]; A -> R [label = ""]; A -> BF [label = ""]; S -> BF [label = ""]; R -> BF [label = ""]; C -> BF [label = ""]; D -> BF [label = ""]; ST -> BF [label = ""]; M -> BF [label = ""]; }

Arbitrating SCSI Bus State Machine

digraph scsi_without_arbitration { fontname="Fira Code" bgcolor="#FFFFFF00" node [fontname="Fira Code"] edge [fontname="Fire Code"] rankdir=LR; node [shape = none]; _ node [shape = doublecircle]; BF node [shape = circle]; _ [label = "RESET"]; BF [label = "BUS FREE"]; S [label = "SELECTION"]; C [label = "COMMAND"]; D [label = "DATA"]; ST [label = "STATUS"]; M [label = "MESSAGE"]; subgraph CDSM { C -> C C -> D C -> ST C -> M D -> D D -> C D -> ST D -> M ST -> ST ST -> C ST -> D ST -> M M -> M M -> C M -> D M -> ST } S -> C [label = ""]; S -> D [label = ""]; S -> ST [label = ""]; S -> M [label = ""]; _ -> BF [label = ""]; BF -> S [label = ""]; S -> BF [label = ""]; C -> BF [label = ""]; D -> BF [label = ""]; ST -> BF [label = ""]; M -> BF [label = ""]; }

Non-Arbitrating SCSI Bus State Machine

The following table lists the timing requirements for each SCSI version.

Name	SCSI1	SCSI2	SCSI3
Arbitration	2.2us	2.4us	2.4us
Assertion	90ns	90ns	Unspecified
Bus Clear	800ns	800ns	800ns
Bus Free	800ns	800ns	800ns
Bus Set	1.8us	1.8us	1.6us
Bus Settle	400ns	400ns	400ns
Cable Skew	10ns	10ns	4ns
Data Release	400ns	400ns	400ns
Deskew	45ns	45ns	45ns
Hold Time	45ns	45ns	Unspecified
Negation	90ns	90ns	Unspecified
Reset Hold	25us	25us	25us
Selection Abort	200us	200us	200us
Selection Timeout	250ms	250ms	250ms
Disconnect	Unspecified	200us	Unspecified
Power to Selection	Unspecified	10s	10s
Reset to Selection	Unspecified	250ms	250ms
Fast Assert	Unspecified	30ns	Unspecified
Fast Cable Skew	Unspecified	5ns	Unspecified
Fast Deskew	Unspecified	20ns	Unspecified
Fast Hold	Unspecified	10ns	Unspecified
Fast Negation	Unspecified	30ns	Unspecified