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NOTE 1 An alternate 16-bit single-cable solution and an alternate 32-bit solution is being defined and the B cable definition will be removed in a future version of SCSI.
Two driver/receiver alternatives are specified:
The single-ended and differential alternatives are mutually exclusive on the same physical bus.
NOTE 2 Use of single-ended drivers and receivers with the fast synchronous data transfer option is not recommended.
NOTE 3 There are successful single-ended implementations using cables with less than 90 ohms characteristic impedance. However, system integrity in single-ended implementations is improved when the characteristic impedance of the cable is greater than 90 ohms. Cable parameters other than characteristic impedance are critical to system integrity. Alternative cable parameters are being investigated as a part of a future version of SCSI.
A minimum conductor size of 0,080 42 mm2 (28 AWG) should be used to minimize noise effects and ensure proper distribution of terminator power. A smaller conductor size may be used for signals other than terminator power.
NOTES
4 To minimize discontinuities and signal reflections, cables of different impedances should not be used in the same bus. Implementations may require trade-offs in shielding effectiveness, cable length, the number of loads, transfer rates, and cost to achieve satisfactory system operation.5 To minimize discontinuities due to local impedance variation, a flat cable should be spaced at least 1,27 mm (0,050 in) from other cables, any other conductor, or the cable itself when the cable is folded.
6 Regulatory agencies may require use of larger wire size.
A stub length of no more than 0,1 m is allowed off the mainline interconnection within any connected equipment or from any connected point.
NOTE 7 Stub clustering should be avoided. Stubs should be spaced at least 0,3 m apart.
SCSI bus termination shall be at each end of the cable and may be internal to the SCSI devices that are at the ends of the cable.
A stub length of no more than 0,2 m is allowed off the mainline interconnection within any connected equipment or from any connected point.
SCSI bus termination shall be at each end of the cable and may be internal to the SCSI devices that are at the ends of the cable.
5.2.2 Differential cable
A 50-conductor flat cable or 25-signal twisted-pair cable shall be used for the A cable. A 68-conductor flat cable or 34-signal twisted-pair cable shall be used for the B cable if the wide SCSI option is implemented. The maximum cumulative cable length shall be 25 m. If twisted-pair cables are used, then twisted pairs in the cable shall be wired to physically opposing contacts in the connector.
NOTE 8 The use of twisted pair cable (either twisted-flat or discrete wire twisted pairs) is strongly recommended. Without twisted pairs, even at slow data rates and very short distances, crosstalk between adjacent signals causes spurious pulses with differential signals.
5.2.3 Cable requirements for fast synchronous data transfer
In systems which use the fast synchronous data transfer option (see 5.8), the A and B cables should meet the conductor size recommendation in 5.2. The cable should have an overall shield suitable for termination in a shielded connector.
In such systems, the cables shall have the following electrical characteristics:
| Characteristic impedance: | 90 ohms to 132 ohms |
| Signal attenuation: | 0,095 dB maximum per metre at 5 Mhz |
| Pair-to-pair propagation delay delta: | 0,20 ns maximum per metre | DC resistance: | 0,230 ohms maximum per metre at 20 degrees C |
The alternative 1 non-shielded high-density SCSI device connector for the A cable (see figure 1) shall be a 50-conductor connector consisting of two rows of 25 female contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The alternative 1 non-shielded high-density cable connector for the A cable (see figure 2) shall be a 50-conductor connector consisting of two rows of 25 male contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The alternative 2 non-shielded low-density SCSI device connector for the A cable (see figure 3) shall be a 50-conductor connector consisting of two rows of 25 male pins with adjacent pins 2,54 mm (0,1 in) apart. A shroud and header body should be used. The non-mating portion of the connector is shown for reference only.
The alternative 2 non-shielded low-density cable connector for the A cable (see figure 4) shall be a 50-conductor connector consisting of two rows of 25 female contacts with adjacent contacts 2,54 mm (0,1 in) apart. It is recommended that keyed connectors be used.
The non-shielded high-density SCSI device connector for the B cable (see figure 1) shall be a 68-conductor connector consisting of two rows of 34 female contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non- mating portion of the connector is shown for reference only.
The non-shielded high-density cable connector for the B cable (see figure 2) shall be a 68-conductor connector consisting of two rows of 34 male contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
In order to support daisy-chain connections, SCSI devices that use shielded connectors should provide two shielded device connectors on the device enclosure. These two connectors may be wired one-to-one with a stub to the SCSI device's drivers and receivers provided the maximum stub length is not violated. Alternatively, two cables may be run from the two shielded connectors to the drivers and receivers so that the maximum stub length is not violated. The length of the cable within the device enclosure is included when calculating the total cable length of the SCSI bus.
The shielded high-density SCSI device connector for the A cable (see figure 5) is a 50-conductor connector consisting of two rows of 25 female contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The shielded high-density cable connector for the A cable (see figure 6) is a 50-conductor connector consisting of two rows of 25 male contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The shielded low-density device connector for the A cable (see figure 7) is a 50-conductor connector consisting of two rows of ribbon contacts spaced 2,16 mm (0,085 in) apart. The non-mating portion of the connector is shown for reference only.
The shielded low-density cable connector for the A cable (see figure 8) is a 50-conductor connector consisting of two rows of ribbon contacts spaced 2,16 mm (0,085 in) apart. The non-mating portion of the connector is shown for reference only.
The shielded high-density SCSI device connector for the B cable (see figure 5) is a 68-conductor connector consisting of two rows of 34 female contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The shielded high-density cable connector for the B cable (see figure 6) is a 68-conductor connector consisting of two rows of 34 male contacts with adjacent contacts 1,27 mm (0,05 in) apart. The non-mating portion of the connector is shown for reference only.
The first termination method above is the same as in SCSI-1. The second termination method is recommended for better signal quality.
5.3.1.1 Non-shielded connector alternative 1 - A cable
5.3.1.2 Non-shielded connector alternative 2 - A cable
5.3.1.3 Non-shielded connector - B cable
Figure 1 - 50/68-contact non-shielded high-density SCSI device connector
(A cable/B cable)
Figure 2 - 50/68-contact non-shielded high-density cable connector
(A cable/B cable)
Figure 3 - 50-contact non-shielded low-density SCSI device connector
(A cable)
Figure 4 - 50-Contact non-shielded low-density cable connector
(A cable)
5.3.2 Shielded connector requirements
Two shielded connector alternatives are specified for the A cable and one shielded connector is specified for the B cable. The connector shielding system should provide a d.c. resistance of less than 10 mohms from the cable shield at its termination point to the SCSI device enclosure.
NOTE 9 SCSI-1 defined three shielded connector systems in an annex. The alternative 1 shielded connector of SCSI-1 has been replaced by a high- density connector in this International Standard. The alternative 2 shielded connector remains unchanged. The EUROCARD boxes shielded connector system of SCSI-1 has been deleted in this International Standard.
5.3.2.1 Shielded connector alternative 1 - A cable
5.3.2.2 Shielded connector alternative 2 - A cable
5.3.2.3 Shielded connector - B cable
Figure 5 - 50/68-contact shielded high-density SCSI device connector
(A cable/B cable)
Figure 6 - 50/68-contact shielded high-density cable connector
(A cable/B cable)
Figure 7 - 50-contact shielded low-density SCSI device connector
Figure 8 - 50-contact shielded low-density cable connector
5.3.3 Connector contact assignments
The connector contact assignments are defined in tables 1 through 5. Table 1 defines which of the other four tables to use and which set of contact assignments to use.
Table 1 - Cross-reference to connector contact assignments
Connector type
Driver/receiver type
Cable
Connector figure
Contact assignment table
Contact set
Non-shielded alternative 1
Single-ended
A
1 & 2
2
2
Non-shielded alternative 1
Single-ended
B
1 & 2
3
Non-shielded alternative 1
Differential
A
1 & 2
4
2
Non-shielded alternative 1
Differential
B
1 & 2
5
Non-shielded alternative 2
Single-ended
A
3 & 4
2
1
Non-shielded alternative 2
Differential
A
3 & 4
4
1
Shielded alternative 1
Single-ended
A
5 & 6
2
2
Shielded alternative 1
Single-ended
B
5& 6
3
Shielded alternative 1
Differential
A
5 & 6
4
2
Shielded alternative 1
Differential
B
5 & 6
5
Shielded alternative 2
Single-ended
A
7 & 8
2
2
Shielded alternative 2
Differential
A
7&8
4
2
Table 2 - Single-ended contact assignments - A cable
Signal name
Connector contact number
Cable conductor number
Connector contact number
Signal name
Set 2
Set 1
Set 1
Set 2
GROUND
1
1
1
2
2
26
-DB(0)
GROUND
2
3
3
4
4
27
-DB(1)
GROUND
3
5
5
6
6
28
-DB(2)
GROUND
4
7
7
8
8
29
-DB(3)
GROUND
5
9
9
10
10
30
-DB(4)
GROUND
6
11
11
12
12
31
-DB(5)
GROUND
7
13
13
14
14
32
-DB(6)
GROUND
8
15
15
16
16
33
-DB(7)
GROUND
9
17
17
18
18
34
-DB(P)
GROUND
10
19
19
20
20
35
GROUND
GROUND
11
21
21
22
22
36
GROUND
RESERVED
12
23
23
24
24
37
RESERVED
OPEN
13
25
25
26
26
38
TERMPWR
RESERVED
14
27
27
28
28
39
RESERVED
GROUND
15
29
29
30
30
40
GROUND
GROUND
16
31
31
32
32
41
-ATN
GROUND
17
33
33
34
34
42
GROUND
GROUND
18
35
35
36
36
43
-BSY
GROUND
19
37
37
38
38
44
-ACK
GROUND
20
39
39
40
40
45
-RST
GROUND
21
41
41
42
42
46
-MSG
GROUND
22
43
43
44
44
47
-SEL
GROUND
23
45
45
46
46
48
-C/D
GROUND
24
47
47
48
48
49
-REQ
GROUND
25
49
49
50
50
50
-I/O
NOTES
Table 3 - Single-ended contact assignments - B cable
Signal name
Connector contact number
Cable conductor number
Connector contact number
Signal name
GROUND
1
1
2
35
GROUND
GROUND
2
3
4
36
-DB(8)
GROUND
3
5
6
37
-DB(9)
GROUND
4
7
8
38
-DB(10)
GROUND
5
9
10
39
-DB(11)
GROUND
6
11
12
40
-DB(12)
GROUND
7
13
14
41
-DB(13)
GROUND
8
15
16
42
-DB(14)
GROUND
9
17
18
43
-DB(15)
GROUND
10
19
20
44
-DB(P1)
GROUND
11
21
22
45
-ACKB
GROUND
12
23
24
46
GROUND
GROUND
13
25
26
47
-REQB
GROUND
14
27
28
48
-DB(16)
GROUND
15
29
30
49
-DB(17)
GROUND
16
31
32
50
-DB(18)
TERMPWRB
17
33
34
51
TERMPWRB
TERMPWRB
18
35
36
52
TERMPWRB
GROUND
19
37
38
53
-DB(19)
GROUND
20
39
40
54
-DB(20)
GROUND
21
41
42
55
-DB(21)
GROUND
22
43
44
56
-DB(22)
GROUND
23
45
46
57
-DB(23)
GROUND
24
47
48
58
-DB(P2)
GROUND
25
49
50
59
-DB(24)
GROUND
26
51
52
60
-DB(25)
GROUND
27
53
54
61
-DB(26)
GROUND
28
55
56
62
-DB(27)
GROUND
29
57
58
63
-DB(28)
GROUND
30
59
60
64
-DB(29)
GROUND
31
61
62
65
-DB(30)
GROUND
32
63
64
66
-DB(31)
GROUND
33
65
66
67
-DB(P3)
GROUND
34
67
68
68
GROUND
NOTES
NOTE 10 An alternate 16-bit single-cable solution and an alternate 32-bit solution is being defined and the B cable definition will be removed in a future version of SCSI.
Table 4 - Differential contact assignments - A cable
Signal name
Connector contact number
Cable conductor number
Connector contact number
Signal name
Set 2
Set 1
Set 1
Set 2
GROUND
1
1
1
2
2
26
GROUND
+DB(0)
2
3
3
4
4
27
-DB(0)
+DB(1)
3
5
5
6
6
28
-DB(1)
+DB(2)
4
7
7
8
8
29
-DB(2)
+DB(3)
5
9
9
10
10
30
-DB(3)
+DB(4)
6
11
11
12
12
31
-DB(4)
+DB(5)
7
13
13
14
14
32
-DB(5)
+DB(6)
8
15
15
16
16
33
-DB(6)
+DB(7)
9
17
17
18
18
34
-DB(7)
+DB(P)
10
19
19
20
20
35
-DB(P)
DIFFSENS
11
21
21
22
22
36
GROUND
RESERVED
12
23
23
24
24
37
RESERVED
TERMPWR
13
25
25
26
26
38
TERMPWR
RESERVED
14
27
27
28
28
39
RESERVED
+ATN
15
29
29
30
30
40
-ATN
GROUND
16
31
31
32
32
41
GROUND
+BSY
17
33
33
34
34
42
-BSY
+ACK
18
35
35
36
36
43
-ACK
+RST
19
37
37
38
38
44
-RST
+ACK
18
35
35
36
36
43
-ACK
+RST
19
37
37
38
38
44
-RST
+MSG
20
39
39
40
40
45
-MSG
+SEL
21
41
41
42
42
46
-SEL
+C/D
22
43
43
44
44
47
-C/D
+REQ
23
45
45
46
46
48
-REQ
+I/O
24
47
47
48
48
49
-I/O
GROUND
25
49
49
50
50
50
GROUND
NOTES
Table 5 - Differential contact assignments - B cable
Signal name
Connector contact number
Cable conductor number
Connector contact number
Signal name
GROUND
1
1
2
35
GROUND
+DB(8)
2
3
4
36
-DB(8)
+DB(9)
3
5
6
37
-DB(9)
+DB(10)
4
7
8
38
-DB(10)
+DB(11)
5
9
10
39
-DB(11)
+DB(12)
6
11
12
40
-DB(12)
+DB(13)
7
13
14
41
-DB(13)
+DB(14)
8
15
16
42
-DB(14)
+DB(15)
9
17
18
43
-DB(15)
+DB(P1)
10
19
20
44
-DB(P1)
+ACKB
11
21
22
45
-ACKB
GROUND
12
23
24
46
DIFFSENS
+REQB
13
25
26
47
-REQB
+DB(16)
14
27
28
48
-DB(16)
+DB(17)
15
29
30
49
-DB(17)
+DB(18)
16
31
32
50
-DB(18)
TERMPWRB
17
33
34
51
TERMPWRB
TERMPWRB
18
35
36
52
TERMPWRB
+DB(19)
19
37
38
53
-DB(19)
+DB(20)
20
39
40
54
-DB(20)
+DB(21)
21
41
42
55
-DB(21)
+DB(22)
22
43
44
56
-DB(22)
+DB(23)
23
45
46
57
-DB(23)
+DB(P2)
24
47
48
58
-DB(P2)
+DB(24)
25
49
50
59
-DB(24)
+DB(25)
26
51
52
60
-DB(25)
+DB(26)
27
53
54
61
-DB(26)
+DB(27)
28
55
56
62
-DB(27)
+DB(28)
29
57
58
63
-DB(28)
+DB(29)
30
59
60
64
-DB(29)
+DB(30)
31
61
62
65
-DB(30)
+DB(31)
32
63
64
66
-DB(31)
+DB(P3)
33
65
66
67
-DB(P3)
GROUND
34
67
68
68
GROUND
NOTE
The conductor number refers to the conductor position when using
0,025 inch centreline flat ribbon cable. Other cable types may be used
to implement equivalent contact assignments.
NOTE 11 An alternate 16-bit single-cable solution and an alternate 32-bit solution is being defined and the B cable definition will be removed in a future version of SCSI.
5.4 Electrical description
For the measurements in this subclause, SCSI bus termination is assumed to be external to the SCSI device. See 5.4.4 for the terminating requirements for the RESERVED lines. SCSI devices may have the provision for allowing optional internal termination.
5.4.1 Single-ended alternative
All signals not defined as RESERVED, GROUND, or TERMPWR shall be terminated at both ends of the cable. The implementor may choose one of the following two methods to terminate each end (see figures 9 and 10):
| VOL (low-level output voltage) | 0,0 to 0,5 V d.c. at 48 mA sinking (signal assertion) |
| VOH (high-level output voltage) | 2,5 to 5,25 V d.c. (signal negation) |
| VIL (low-level input voltage) | 0,0 V d.c. to 0,8 V d.c. (signal true) |
| VIH (high-level input voltage) | 2,0 V d.c. to 5,25 V d.c. (signal false) |
| IIL (low-level input current) | -0,4 mA to 0,0 mA at VI = 0,5 V d.c. |
| IIH (high-level input current) | 0,0 mAto 0,1 mA at VI = 2,7 V d.c. |
| Minimum input hysteresis | 0,2 V d.c. |
| Maximum input capacitance | 25 pF (measured at the device connector closest to the stub, if any, within the device) |
To achieve maximum noise immunity and to assure proper operation with complex cable configurations, it is recommended that the nominal switching threshold be approximately 1,4 V.
5.4.2 Differential alternative
All signals consist of two lines denoted +SIGNAL and -SIGNAL. A signal is true when +SIGNAL is more positive than -SIGNAL, and a signal is false when -SIGNAL is more positive than +SIGNAL. All assigned signals of the A and B cables described in 5.6 shall be terminated at each end of the cable with a terminator network as shown in figure 11. Resistor tolerances in the terminator network shall be +/-5 % or less.
The DIFFSENS signal of the connector is used as an active high enable for the differential drivers. If a single-ended device or terminator is inadvertently connected, this signal is grounded, disabling the differential drivers (see figure 12).
The characteristic impedance of differential terminators is 122 ohms.
| VOL (low-level output voltage) | 1,7 V maximum at IOL (low-level output current) = 55 mA. |
| VOH (high-level output voltage) | 2,7 V minimum at IOH (high-level output current)= -55 mA. |
| VOD (differential output voltage) | 1,0 V minimum with common-mode voltage ranges from -7 V d.c. to +12 V d.c. |
The output characteristics shall additionally conform to EIA RS-485-1983.
| II (input current on either input) | +/- 2,0 mA maximum. |
| Maximum input capacitance | 25 pF. |
The input characteristics shall additionally conform to EIA RS-485-1983.
All terminators independent of location shall be powered from the TERMPWR and TERMPWRB contact(s). The use of keyed connectors is recommended in SCSI devices that provide terminator power to prevent accidental grounding or the incorrect connection of terminator power.
SCSI devices shall sink no more than 1,0 mA from TERMPWR and no more than 1,0 mA from TERMPWRB except to power an optional internal terminator.
Single-ended SCSI devices providing terminator power on cable A shall have the following characteristics:
When two SCSI devices communicate on the SCSI bus, one acts as an initiator and the other acts as a target. The initiator originates an operation and the target performs the operation. An SCSI device usually has a fixed role as an initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices that are connected to a target. The target may be physically housed within the peripheral device in which case the peripheral device is referred to as an embedded SCSI device.
Certain SCSI bus functions are assigned to the initiator and certain SCSI bus functions are assigned to the target. The initiator may arbitrate for the SCSI bus and select a particular target. The target may request the transfer of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some cases it may arbitrate for the SCSI bus and reselect an initiator for the purpose of continuing an operation.
Information transfers on the DATA BUS are asynchronous and follow a defined REQ/ACK handshake protocol. One byte of information may be transferred with each handshake on the A cable and, if the wide data transfer option is implemented, one or three bytes of information may be transferred with each handshake on the B cable. An option is defined for synchronous data transfer.
For the first condition above, the maximum time for an SCSI device to clear the bus is 1200 nanoseconds from BSY and SEL first becoming both false. If an SCSI device requires more than a bus settle delay to detect BUS FREE phase, it shall clear the bus within a bus clear delay minus the excess time.
5.4.3 Terminator power
SCSI initiators shall supply terminator power to the TERMPWR contact(s) and, if it implements the wide SCSI option, to the TERMPWRB contacts. This power shall be supplied through a diode or similar semiconductor that prevents backflow of power to the SCSI device. Targets and SCSI devices that become temporary initiators (e.g. targets which implement the COPY command or asynchronous event notification) are not required to supply terminator power. Any SCSI device may supply terminator power. Interface error rates are lower if the termination voltage is maintained at the extreme ends of the cable.
NOTE 12 Regulatory agencies may require limiting maximum (short circuit) current to the terminator power lines. Recommended current limiting is 1,5 A for TERMPWR and 2 A for TERMPWRB. For systems utilizing multiple initiators, the initiators may be configured with option straps or current limiting devices. Maximum available current should not exceed 5 A.
VTerm = 4,25 V d.c. to 5,25 V d.c. 900 mA minimum source drive capability
Differential SCSI devices providing terminator power on cable A shall have the following characteristics:
VTerm = 4,0 V d.c. to 5,25 V d.c. 600 mA minimum source drive capability
Single-ended SCSI devices providing terminator power on cable B shall have the following characteristics:
VTerm = 4,5 V d.c. to 5,25 V d.c. 1500 mA minimum source drive capability
Differential SCSI devices providing terminator power on cable B shall have the following characteristics:
VTerm = 4,0 V d.c. to 5,25 V d.c. 1000 mA minimum source drive capability
NOTE 13 It is recommended that the terminator power lines be decoupled at each terminator with at least a 2,2 uF high-frequency capacitor to improve signal quality.
Figure 9 - Alternative 1 termination for single-ended devices
Figure 10 - Alternative 2 termination for single-ended devices
Figure 11 - Termination for differential devices
Figure 12 - Differential driver protection circuit
5.4.4 RESERVED lines
The lines labelled RESERVED in the A cable contact assignment tables (table 2 and table 4) shall be connected to ground in the bus terminator assemblies or in the end devices on the SCSI cable. The RESERVED lines should be open in the other SCSI devices, but may be connected to ground.
5.5 SCSI bus
Communication on the SCSI bus is allowed between only two SCSI devices at any given time. There is a maximum of eight SCSI devices. Each SCSI device has an SCSI ID bit assigned as shown in figure 13. Three sample system configurations are shown in figure 14. There can be any combination of initiators and targets provided there is at least one of each.
Figure 14 - Sample SCSI configurations
5.6 SCSI bus signals
There are a total of 18 signals on the A cable and 29 signals on the B cable. A total of 11 signals are used for control and 36 are used for data (messages, commands, status and data), including parity. These signals are described as follows:
NOTE 14 The SEL signal was not defined as OR-tied in SCSI-1. It has been defined as OR-tied in SCSI-2 in anticipation of needing another OR-tied signal for future standardization. This does not cause an operational problem in mixing SCSI-1 and SCSI-2 devices.
5.6.1 Signal values
Signals may assume true or false values. There are two methods of driving these signals. In both cases, the signal shall be actively driven true, or asserted. In the case of OR-tied drivers, the driver does not drive the signal to the false state, rather the bias circuitry of the bus terminators pulls the signal false whenever it is released by the drivers at every SCSI device.
If any driver is asserted, then the signal is true. In the case of non-OR-tied drivers, the signal may be actively driven false. In this International Standard, wherever the term negated is used, it means that the signal may be actively driven false, or may be simply released (in which case the bias circuitry pulls it false), at the option of the implementor.
The advantage to actively driving signals false during information transfer is that the transition from true to false occurs more quickly and the noise margin is much higher than if the signal is simply released. This facilitates reliable data transfer at high rates, especially at the longer cable lengths used with differential drivers.
5.6.2 OR-tied signals
The BSY, SEL, and RST signals shall be OR-tied only. In the ordinary operation of the bus, the BSY and RST signals may be simultaneously driven true by several drivers. No signals other than BSY, RST, and DB(P) are simultaneously driven by two or more drivers, and any signal other than BSY, SEL, and RST may employ OR-tied or non-OR-tied drivers. DB(P) shall not be driven false during the ARBITRATION phase but may be driven false in other phases. There is no operational problem in mixing OR-tied and non-OR-tied drivers on signals other than BSY and RST.
5.6.3 Signal sources
Table 6 indicates which type of SCSI device is allowed to source each signal. No attempt is made to show if the source is driving asserted, driving negated, or is passive. All SCSI device drivers that are not active sources be in the passive state. The RST signal may be asserted by any SCSI device at any time.
Table 6 - Signal sources
Bus phase
A cable signals
B cable signals
BSY
SEL
C/D, I/O, MSG, REQ
ACK, ATN
DB(7-0) DB(P)
REQB
ACKB
DB(31-8) DB(P1) DB(P2) DB(P3)
BUS FREE
None
None
None
None
None
None
None
None
ARBITRATION
All
Win
None
None
S ID
None
None
None
SELECTION
I&T
Init
None
Init
Init
None
None
None
RESELECTION
I&T
Targ
Targ
Init
Targ
None
None
None
COMMAND
Targ
None
Targ
Init
Init
None
None
None
DATA IN
Targ
None
Targ
Init
Targ
Targ
Init
Targ
DATA OUT
Targ
None
Targ
Init
Init
Targ
Init
Init
STATUS
Targ
None
Targ
Init
Targ
None
None
None
MESSAGE IN
Targ
None
Targ
Init
Targ
None
None
None
MESSAGE OUT
Targ
None
Targ
Init
Init
None
None
None
All:
The signal shall be driven by all SCSI devices that
are actively arbitrating.
S ID:
A unique data bit (the SCSI ID) shall be driven by each SCSI device
that is actively arbitrating; the other seven data bits shall be
released (i.e., not driven) by this SCSI device. The parity bit
(DB(P)) may be released or driven to the true state, but shall never
be driven to the false state during this phase.
I&T:
The signal shall be driven by the initiator, target, or both, as
specified in the SELECTION phase and RESELECTION phase.
Init:
If driven, this signal shall be driven only by the active initiator.
None:
The signal shall be released; that is, not be driven by any SCSI
device. The bias circuitry of the bus terminators pulls the signal
to the false state.
Win:
The signal shall be driven by the one SCSI device that wins
arbitration.
Targ:
If the signal is driven, it shall be driven only by the active
target.
5.7 SCSI bus timing
Unless otherwise indicated, the delay-time measurements for each SCSI device, shown in table 7, shall be calculated from signal conditions existing at that SCSI device's own SCSI bus connection. Thus, these measurements (except cable skew delay) can be made without considering delays in the cable. The timing characteristics of each signal are described in the following paragraphs.
Table 7 - SCSI bus timing values
Timing description
Timing value
Arbitration delay
2,4 us
Assertion period
90 ns
Bus clear delay
800 ns
Bus free delay
800 ns
Bus set delay
1,8 us
Bus settle delay
400 ns
Cable skew delay
10 ns
Data release delay
400 ns
Deskew delay
45 ns
Disconnection delay
200 us
Hold time
45 ns
Negation period
90 ns
Power-on to selection time
10 s recommended
Reset to selection time
250 ms recommended
Reset hold time
25 us
Selection abort time
200 us
Selection time-out delay
250 ms recommended
Transfer period
set during an SDTR message
Fast assertion period
30 ns
Fast cable skew delay
5 ns
Fast deskew delay
20 ns
Fast hold time
10 ns
Fast negation period
30 ns
5.7.1 Arbitration delay
The minimum time an SCSI device shall wait from asserting BSY for arbitration until the DATA BUS can be examined to see if arbitration has been won. There is no maximum time.
5.7.2 Assertion period
The minimum time that a target shall assert REQ (or REQB) while using synchronous data transfers. Also, the minimum time that an initiator shall assert ACK (or ACKB) while using synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
5.7.3 Bus clear delay
The maximum time for an SCSI device to stop driving all bus signals after:
5.7.4 Bus free delay
The minimum time that an SCSI device shall wait from its detection of the BUS FREE phase (see 6.1.1) until its assertion of BSY when going to the ARBITRATION phase.
5.7.5 Bus set delay
The maximum time for an SCSI device to assert BSY and its SCSI ID bit on the DATA BUS after it detects BUS FREE phase (see 6.1.1) for the purpose of entering the ARBITRATION phase.
5.7.6 Bus settle delay
The minimum time to wait for the bus to settle after changing certain control signals as called out in the protocol definitions.
5.7.7 Cable skew delay
The maximum difference in propagation time allowed between any two SCSI bus signals measured between any two SCSI devices.
5.7.8 Data release delay
The maximum time for an initiator to release the DATA BUS signals following the transition of the I/O signal from false to true.
5.7.9 Deskew delay
The minimum time required for deskew of certain signals.
5.7.10 Disconnection delay
The minimum time that a target shall wait after releasing BSY before participating in an ARBITRATION phase when honouring a DISCONNECT message from the initiator.
5.7.11 Hold time
The minimum time added between the assertion of REQ (or REQB) or ACK (or ACKB) and the changing of the data lines to provide hold time in the initiator or target while using synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
5.7.12 Negation period
The minimum time that a target shall negate REQ (or REQB) while using synchronous data transfers. Also, the minimum time that an initiator shall negate ACK (or ACKB) while using synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
5.7.13 Power-on to selection time
The recommended maximum time from power application until an SCSI target is able to respond with appropriate status and sense data to the TEST UNIT READY, INQUIRY, and REQUEST SENSE commands.
5.7.14 Reset to selection time
The recommended maximum time after a hard RESET condition until an SCSI target is able to respond with appropriate status and sense data to the TEST UNIT READY, INQUIRY, and REQUEST SENSE commands.
5.7.15 Reset hold time
The minimum time for which RST is asserted. There is no maximum time.
5.7.16 Selection abort time
The maximum time that a target (or initiator) shall take from its most recent detection of being selected (or reselected) until asserting a BSY response. This time-out is required to ensure that a target (or initiator) does not assert BSY after a SELECTION (or RESELECTION) phase has been aborted. This is not the selection time-out period; see 6.1.3.1 and 6.1.4.2 for a complete description.
5.7.17 Selection time-out delay
The minimum time that an SCSI device should wait for a BSY response during the SELECTION or RESELECTION phase before starting the time-out procedure.
NOTE 15 The selection time-out delay is only a recommended time period.
5.7.18 Transfer period
The minimum time allowed between the leading edges of successive REQ pulses or of successive ACK pulses while using synchronous data transfers. (See 6.1.5.2 and 6.6.21)
5.8 Fast synchronous transfer option
When devices negotiate a synchronous data transfer period of less than 200 ns they are said to be using fast synchronous data transfers. Devices that negotiate a synchronous data transfer period greater than or equal to 200 ns use timing parameters specified in 5.7. When a fast synchronous data transfer period is negotiated, those specific times redefined in this section are used; those not redefined remain the same. The minimum synchronous data transfer period is 100 ns.
5.8.1 Fast assertion period
The minimum time that a target shall assert REQ (or REQB) while using fast synchronous data transfers. It is also the minimum time that an initiator shall assert ACK (or ACKB) while using fast synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
5.8.2 Fast cable skew delay
The maximum difference in propagation time allowed between any two SCSI bus signals measured between any two SCSI devices while using fast synchronous data transfers.
5.8.3 Fast deskew delay
The minimum time required for deskew of certain signals while using fast synchronous data transfers.
5.8.4 Fast hold time
The minimum time added between the assertion of REQ (or REQB) or ACK (or ACKB) and the changing of the data lines to provide hold time in the initiator or target while using fast synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
5.8.5 Fast negation period
The minimum time that a target shall negate REQ (or REQB) while using fast synchronous data transfers. Also, the minimum time that an initiator shall negate ACK (or ACKB) while using fast synchronous data transfers. REQB and ACKB timings only apply to optional wide data transfers.
