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.
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.
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.
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)
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.
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.
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.
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
+==========================-============-=====-=========-==========-=======+ | | Driver/ | | | Contact | | | | receiver | |Connector|assignment|Contact| | Connector type | type |Cable| figure | table | 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 | +==========================================================================+
+================-===============-=========-===============-================+ | | Connector | | Connector | | | | contact number| Cable |contact number | | | Signal |---------------|conductor|---------------| Signal | | name | Set 2 | Set 1 | number | Set 1 | Set 2 | name | |----------------+-------+-------+---------+-------+-------+----------------| | 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 | | 1 The minus sign next to a signal indicates active low. | | 2 The conductor number refers to the conductor position when using | | 0,050 inch centreline flat ribbon cable with a low-density connector or| | when using 0,025 inch centreline flat ribbon cable with a high-density | | connector. Other cable types may be used to implement equivalent | | contact assignments. | | 3 Two sets of contact assignments are shown. Refer to table 1 to | | determine which set of contacts applies to each connector. | | 4 See 4.4.4 for a definition of the RESERVED lines. | +===========================================================================+
+================-===============-=========-===============-================+ | | Connector | Cable | Connector | | | Signal | contact |conductor| contact | Signal | | name | number | number | number | 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 | | 1 The minus sign next to a signal indicates active low. | | 2 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 contact assignments. | +===========================================================================+
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.
+================-===============-=========-===============-================+ | | Connector | | Connector | | | |contact number | Cable |contact number | | | Signal |---------------|conductor|---------------| Signal | | name | Set 2 | Set 1 | number | Set 1 | Set 2 | name | |----------------+-------+-------+---------+-------+-------+----------------| | 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 | | 1 The conductor number refers to the conductor position when using | | 0,050 inch centreline flat ribbon cable with a low-density connector or| | when using 0,025 inch centreline flat ribbon cable with a high-density | | connector. Other cable types may be used to implement equivalent | | contact assignments. | | 2 Two sets of contact assignments are shown. Refer to table 1 to | | determine which set of contacts applies to each connector. | | 3 See 5.4.4 for a definition of the RESERVED lines. | +===========================================================================+
+================-===============-=========-===============-================+ | | Connector | Cable | Connector | | | Signal | contact |conductor| contact | Signal | | name | number | number | number | 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.
The first termination method above is the same as in SCSI-1. The second termination method is recommended for better signal quality.
All signals shall use open-collector or three-state drivers. Each signal driven by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector:
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)
SCSI devices with power on shall meet the following electrical characteristics on each signal (including both receivers and passive drivers):
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)
It is recommended that SCSI devices with power off also meet the above IIL and IIH electrical characteristics on each signal.
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.
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.
Each signal driven by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector:
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.
VOL and VOH shall be as measured between the output terminal and the SCSI device's logic ground reference.
The output characteristics shall additionally conform to EIA RS-485-1983.
SCSI devices shall meet the following electrical characteristics on each signal (including both receivers and passive drivers):
II (input current on either input) = +/- 2,0 mA maximum. Maximum input capacitance = 25 pF.The II requirement shall be met with the input voltage varying between -7 V d.c. and +12 V d.c., with power on or off, and with the hysteresis equaling 35 mv, minimum.
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.
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.
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:
VTerm = 4,25 V d.c. to 5,25 V d.c. 900 mA minimum source drive capabilityDifferential 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 capabilitySingle-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 capabilityDifferential 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
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.
Figure 14 - Sample SCSI configurations
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.
+==============-=====================================-========================+ | Bus phase | A cable signals | B cable signals | | |-------------------------------------+------------------------| | | | | C/D, | | | | | DB(31-8) | | | | | I/O, | | | | | DB(P1) | | | | | MSG, | ACK, | DB(7-0) | | | DB(P2) | | | BSY | SEL | REQ | ATN | DB(P) | REQB | ACKB | 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. | +=============================================================================+
+==============================-===================================+ | 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 | +==================================================================+
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.
NOTE 15 The selection time-out delay is only a recommended time period.
