Abstract: This article describes the differences between SCSI, FAST SCSI, and ULTRA SCSI, and gives the connection of the DS2120 SCSI terminator and the required level. This article also discusses the MSTR / SLV functions related to DIFF_CAP and DIFFSENS.
Please also refer to: SCSI Terminator Webpage
When choosing a SCSI (Small Computer System Interface) transceiver, whether it is a single-ended (SE) or differential (HVD or LVD) device, determine the transmission data rate, cable length, and the maximum number of devices the host controller can support.
The SCSI¹ standard (now called SCSI-1) was released in 1981. It uses a proprietary single-ended, 8-bit bus transceiver to transfer data between the host and peripheral devices. SCSI-1 defines the data transfer rate as 5M bytes per second. After the publication of this standard, SCSI technology has developed rapidly. SCSI-2 specifies a data rate of 10M bytes per second, and has a selectable high voltage differential signal (HVD) drive and a 16-bit wide byte data bus. SCSI-3 SPI ™ extends the rate to 20M bytes per second, using a wide byte HVD bus. With the introduction of Ultra SCSI, Ultra2, Ultra160 and Ultra320 standards in the future, the data bus transmission speed has reached 320M bytes per second. (Refer to Table 1.0)
Table 1. SCSI terms and definitions approved by STA
note:
(1) In point-to-point or engineering applications, the maximum bus length listed in the table may be exceeded.
(2) Using a "narrow" byte bus, the above SCSI, Ultra SCSI, or Ultra2 SCSI are optional.
(3) LVD did not give a definition of this rate in the original SCSI standard. If all devices on the bus support LVD, there is no problem with supporting a length of 12 meters at this speed. If all the devices on the bus are single-ended, they can reach 25 meters in point-to-point applications.
(4) There is no single-ended definition for devices whose speed exceeds the Ultra standard.
(5) There is no definition of HVD for devices whose speed exceeds Ultra2 standard.
(6) Starting with Ultra2, all new speed versions use a wide bus.
HVD SCSI is an integral part of the SCSI-2 standard. It can increase the data transmission speed, extend the transmission distance, and increase the maximum number of devices on the bus. HVD increased the cable length to 25 meters (82 feet), and the wide-byte Fast SCSI bus allowed 16 SCSI devices (including host adapters) to be attached to the bus. However, HVD SCSI requires a high-power transceiver, which uses differential voltage signals to indicate the logic state on the bus. A pair of parallel voltage signals (a positive voltage and a negative voltage) are transmitted along the bus to establish a differential voltage. If (V +)-(V-) is a positive number, the detection result at the receiving end is logic "1". If (V +)-(V-) is a negative number, the detection result of the receiving end is logic "0". Differential buses have smaller noise and ground potential deviations than single-ended buses, because any bus interference acts on V + and V- at the same time, and appears as a mode interference, which can be effectively suppressed. It is for this reason that the differential bus is also called a balanced bus. This form of bus can extend the cable length and reduce noise. However, the high-power transceiver of the balanced bus cannot be integrated into one or two control chips, which makes most price-sensitive applications unable to adopt the differential bus method.
LVD SCSI was released in 1995. It is a low-power, low-cost differential bus transceiver that guarantees signal integrity and has the same high-speed data transmission capabilities as HVD SCSI. The LVD (V + and V-) signal swing is about ± 400mV and swings around a common-mode bias voltage of 1.25V. Compared with single-ended or high-voltage differential signals, the smaller LVD signal swing can quickly meet the requirements Logical state. Using the LVD SCSI based on the EIA-644 (LVDS) copper wire data transmission standard can achieve a data rate of 655 Mbytes per second (point-to-point transmission). Another advantage of LVD SCSI over HVD SCSI is that it is backward compatible to the SE SCSI² standard. For Ultra3, Ultra160 and higher speed standards, neither SE nor HVD define speed and distance parameters. For the standards above Ultra160, only LVD gives the exact definition, refer to Table 1.0.
The DS2120 LVD SCSI terminator fully meets the requirements of the Ultra320, Ultra3, Ultra160 and Ultra2 (LVD only) SCSI interface standards. Because the DS2120 is just an LVD SCSI terminator, if the SE or HVD device is connected to the bus, the DS2120 will automatically leave the bus. The device internally detects the voltage on the DIFFSENS line to achieve this function. The LVD terminal provides two lithographic resistors with a 5% tolerance biased by a current source and a common-mode voltage source generated by a 1.25V bandgap reference. A Y-terminal configuration is used, with 105Ω differential impedance and 150Ω common mode impedance. When no driver is connected to the LVD SCSI bus, it will maintain a 112mV fail-safe bias voltage. Another fail-safe measure is a band-gap temperature-to-voltage converter used to detect whether the device temperature exceeds 150 ° C. Once the thermal shutdown temperature threshold set by the device is reached or exceeded, the DS2120 will be automatically isolated from the bus.
The DIFF_CAP pin monitors the DIFFSENS line to ensure that the device is operating in the proper mode. If the DIFF_CAP pin voltage is between 0.7V and 1.9V, the device enters the LVD operating state after the mode conversion delay. Wide-byte SCSI applications require three DS2120 LVD termination devices (see Figure 1.0). In typical applications, SCSI devices are connected using the following daisy chain. When MSTR / SLV is high, DIFFSENS can drive the SCSI DIFFSENS line to determine the appropriate operating mode.
The device has a 75kΩ pull-up resistor between the MSTR / SLV and TPWR pins. Because of the internal pull-up resistor, the MSTR / SLV pin can be connected to a high level (see Figure 1.0), or it can be left floating ³.
figure 1.
DS2120 is a high-performance LVD SCSI terminator. It ensures that the system power consumption and system cost are not increased while meeting the speed requirements of the HVD SCSI terminator. In the above configuration, only one set of terminators resides in the host bus adapter (HBA), and the other terminators are placed in the small connectors of the SCSI cable termination. The DS2120E is available in a small size, 28-pin, 4.4mm TSSOP package. This small size package allows it to be easily integrated into cable terminations or cable adapters. The DS2120 also has a shutdown capacitance as low as 3pF, which allows the device to be hot-swapped and operates from a single 2.7V to 5.5V power supply. The operating temperature range is 0 ° C to + 70 ° C.
¹ Please refer to the SCSI Industry Association (STA) web page: http: //
² Any version of SCSI definitions, including LVD SCSI, must be backward compatible. The new standard uses a detector, which is called a differential line and uses a signal line called DIFFSENSE to distinguish terminal devices of different standards. The terminal device outputs a signal voltage to the DIFFSENSE signal line, and uses this signal voltage to distinguish SE, HVD and LVD. A SE bus whose DIFFSENSE voltage value is less than 0.5V, an LVD bus DIFFSENSE voltage value is between 0.7V and 1.9V, and an HVD bus is higher than 2.4V.
³ Please refer to Application Note 510: MSTR / SLV Link Configuration of SCSI Bus.
Please also refer to: SCSI Terminator Webpage
When choosing a SCSI (Small Computer System Interface) transceiver, whether it is a single-ended (SE) or differential (HVD or LVD) device, determine the transmission data rate, cable length, and the maximum number of devices the host controller can support.
The SCSI¹ standard (now called SCSI-1) was released in 1981. It uses a proprietary single-ended, 8-bit bus transceiver to transfer data between the host and peripheral devices. SCSI-1 defines the data transfer rate as 5M bytes per second. After the publication of this standard, SCSI technology has developed rapidly. SCSI-2 specifies a data rate of 10M bytes per second, and has a selectable high voltage differential signal (HVD) drive and a 16-bit wide byte data bus. SCSI-3 SPI ™ extends the rate to 20M bytes per second, using a wide byte HVD bus. With the introduction of Ultra SCSI, Ultra2, Ultra160 and Ultra320 standards in the future, the data bus transmission speed has reached 320M bytes per second. (Refer to Table 1.0)
Table 1. SCSI terms and definitions approved by STA
| STA Terms | Bus Speed, MBytes / Sec. Max. | Bus Width, bits | Max. Bus Lengths, Meters (1) | Max. Device Support | ||
| Single- Ended | LVD | HVD | ||||
| SCSI-1 (2) | 5 | 8 | 6 | (3) | 25 | 8 |
| Fast SCSI (2) | 10 | 8 | 3 | (3) | 25 | 8 |
| Fast Wide SCSI | 20 | 16 | 3 | (3) | 25 | 16 |
| Ultra SCSI (2) | 20 | 8 | 1.5 | (3) | 25 | 8 |
| Ultra SCSI (2) | 20 | 8 | 3 | 4 | ||
| Wide Ultra SCSI | 40 | 16 | (3) | 25 | 16 | |
| Wide Ultra SCSI | 40 | 16 | 1.5 | 8 | ||
| Wide Ultra SCSI | 40 | 16 | 3 | 4 | ||
| Ultra2 SCSI (2,4) | 40 | 8 | (4) | 12 | 25 | 8 |
| Wide Ultra2 SCSI (4) | 80 | 16 | (4) | 12 | 25 | 16 |
| Ultra3 SCSI or Ultra160 SCSI (6) | 160 | 16 | (4) | 12 | (5) | 16 |
| Ultra320 SCSI (6) | 320 | 16 | (4) | 12 | (5) | 16 |
note:
(1) In point-to-point or engineering applications, the maximum bus length listed in the table may be exceeded.
(2) Using a "narrow" byte bus, the above SCSI, Ultra SCSI, or Ultra2 SCSI are optional.
(3) LVD did not give a definition of this rate in the original SCSI standard. If all devices on the bus support LVD, there is no problem with supporting a length of 12 meters at this speed. If all the devices on the bus are single-ended, they can reach 25 meters in point-to-point applications.
(4) There is no single-ended definition for devices whose speed exceeds the Ultra standard.
(5) There is no definition of HVD for devices whose speed exceeds Ultra2 standard.
(6) Starting with Ultra2, all new speed versions use a wide bus.
HVD SCSI is an integral part of the SCSI-2 standard. It can increase the data transmission speed, extend the transmission distance, and increase the maximum number of devices on the bus. HVD increased the cable length to 25 meters (82 feet), and the wide-byte Fast SCSI bus allowed 16 SCSI devices (including host adapters) to be attached to the bus. However, HVD SCSI requires a high-power transceiver, which uses differential voltage signals to indicate the logic state on the bus. A pair of parallel voltage signals (a positive voltage and a negative voltage) are transmitted along the bus to establish a differential voltage. If (V +)-(V-) is a positive number, the detection result at the receiving end is logic "1". If (V +)-(V-) is a negative number, the detection result of the receiving end is logic "0". Differential buses have smaller noise and ground potential deviations than single-ended buses, because any bus interference acts on V + and V- at the same time, and appears as a mode interference, which can be effectively suppressed. It is for this reason that the differential bus is also called a balanced bus. This form of bus can extend the cable length and reduce noise. However, the high-power transceiver of the balanced bus cannot be integrated into one or two control chips, which makes most price-sensitive applications unable to adopt the differential bus method.
LVD SCSI was released in 1995. It is a low-power, low-cost differential bus transceiver that guarantees signal integrity and has the same high-speed data transmission capabilities as HVD SCSI. The LVD (V + and V-) signal swing is about ± 400mV and swings around a common-mode bias voltage of 1.25V. Compared with single-ended or high-voltage differential signals, the smaller LVD signal swing can quickly meet the requirements Logical state. Using the LVD SCSI based on the EIA-644 (LVDS) copper wire data transmission standard can achieve a data rate of 655 Mbytes per second (point-to-point transmission). Another advantage of LVD SCSI over HVD SCSI is that it is backward compatible to the SE SCSI² standard. For Ultra3, Ultra160 and higher speed standards, neither SE nor HVD define speed and distance parameters. For the standards above Ultra160, only LVD gives the exact definition, refer to Table 1.0.
The DS2120 LVD SCSI terminator fully meets the requirements of the Ultra320, Ultra3, Ultra160 and Ultra2 (LVD only) SCSI interface standards. Because the DS2120 is just an LVD SCSI terminator, if the SE or HVD device is connected to the bus, the DS2120 will automatically leave the bus. The device internally detects the voltage on the DIFFSENS line to achieve this function. The LVD terminal provides two lithographic resistors with a 5% tolerance biased by a current source and a common-mode voltage source generated by a 1.25V bandgap reference. A Y-terminal configuration is used, with 105Ω differential impedance and 150Ω common mode impedance. When no driver is connected to the LVD SCSI bus, it will maintain a 112mV fail-safe bias voltage. Another fail-safe measure is a band-gap temperature-to-voltage converter used to detect whether the device temperature exceeds 150 ° C. Once the thermal shutdown temperature threshold set by the device is reached or exceeded, the DS2120 will be automatically isolated from the bus.
The DIFF_CAP pin monitors the DIFFSENS line to ensure that the device is operating in the proper mode. If the DIFF_CAP pin voltage is between 0.7V and 1.9V, the device enters the LVD operating state after the mode conversion delay. Wide-byte SCSI applications require three DS2120 LVD termination devices (see Figure 1.0). In typical applications, SCSI devices are connected using the following daisy chain. When MSTR / SLV is high, DIFFSENS can drive the SCSI DIFFSENS line to determine the appropriate operating mode.
The device has a 75kΩ pull-up resistor between the MSTR / SLV and TPWR pins. Because of the internal pull-up resistor, the MSTR / SLV pin can be connected to a high level (see Figure 1.0), or it can be left floating ³.
figure 1.
DS2120 is a high-performance LVD SCSI terminator. It ensures that the system power consumption and system cost are not increased while meeting the speed requirements of the HVD SCSI terminator. In the above configuration, only one set of terminators resides in the host bus adapter (HBA), and the other terminators are placed in the small connectors of the SCSI cable termination. The DS2120E is available in a small size, 28-pin, 4.4mm TSSOP package. This small size package allows it to be easily integrated into cable terminations or cable adapters. The DS2120 also has a shutdown capacitance as low as 3pF, which allows the device to be hot-swapped and operates from a single 2.7V to 5.5V power supply. The operating temperature range is 0 ° C to + 70 ° C.
¹ Please refer to the SCSI Industry Association (STA) web page: http: //
² Any version of SCSI definitions, including LVD SCSI, must be backward compatible. The new standard uses a detector, which is called a differential line and uses a signal line called DIFFSENSE to distinguish terminal devices of different standards. The terminal device outputs a signal voltage to the DIFFSENSE signal line, and uses this signal voltage to distinguish SE, HVD and LVD. A SE bus whose DIFFSENSE voltage value is less than 0.5V, an LVD bus DIFFSENSE voltage value is between 0.7V and 1.9V, and an HVD bus is higher than 2.4V.
³ Please refer to Application Note 510: MSTR / SLV Link Configuration of SCSI Bus.
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