ask THE SCSI EXPERT.Visit the SCSI SCSI in full Small Computer System Interface Once common standard for connecting peripheral devices (disks, modems, printers, etc.) to small and medium-sized computers. SCSI has given way to faster standards, such as Firewire and USB. Trade Association's web site (www.scsita.org) to ask the SCSI Expert a question. Q: Would a device with an Ultral60 SCSI interface SCSI interface - SCSI adaptor be compatible with a Wide Ultra SCSI The designation for various high-speed SCSI interfaces. The original specification was Ultra SCSI, followed by Ultra2, Ultra3, etc. For details, see SCSI. host adapter Also called a "controller" or "host bus adapter," it is a device that connects one or more peripheral units to a computer. It is typically an expansion card that plugs into the bus. IDE and SCSI are examples of peripheral interfaces that call their controllers host adapters. See host. card? A: Yes a device with an Ultral60 SCSI interface can be placed on a Wide Ultra SCSI host adapter as long as the Ultra SCSI host adapter is single ended (hardware) single ended - An electrical connection where one wire carries the signal and another wire or shield is connected to electrical ground. This is in contrast to a differential connection where the second wire carries an inverted signal. (SE) and not differential (HVD (1) (High Voltage Differential) See SCSI and differential signaling. (2) (Holographic Versatile Disc) A high-capacity optical disc from the HVD Forum (www.hvd-forum. ). If the Ultra SCSI host adapter is HVD you will need to use a SCSI bus expander called an HVD-to-SE converter or an HVD-to-LVD converter. The data throughput and cable lengths to the Ultral 60 SCSI device will be limited to the data throughput and cable length capability of the Ultra SCSI host adapter. Q: What restrictions do I have running a differential SCSI DLT (Digital Linear Tape) A magnetic tape technology originally developed by Digital for its VAX line. The technology was later sold to Quantum, which makes it available to other manufacturers. DLT uses half-inch, single-hub cartridges similar to IBM's 3480/3490/3590 line. drive with any other peripherals I might have? Is it possible to mix differential and SE devices on the same chain? If so, what performance can I expect? A: As you have no doubt discovered, the HVD differential bus is not compatible with either the single ended (SE) bus or the new LVD See LVDS. LVD - Low Voltage Differential differential bus. In order to interconnect devices with these different SCSI interfaces, you will need a device called a SCSI Expander. SCSI Expanders can provide conversion. For instance, if you want to place your differential (HVD) DLT drive on a single ended SCSI bus, you can connect an SE-to-HVD converter between them. A correctly designed converter will not reduce the data throughput. As in any SCSI data transfer, the data throughput will be determined by the slower of the two devices involved in the SCSI data transfer. The converter does not consume a SCSI ID and is completely transparent to the hardware, software, and the operator. Also, you can have the full SE cable length as well as the full HVD cable length. Q: Assuming all devices on the chain are the same bus type and speed, is there a difference in performance (speed) between those SCSI devices attached inside the computer to those attached externally? A: There is no difference in internal or external performance, all other things being equal. Q: Assuming that two computers can share the same SCSI bus (with due regard being given to termination requirements and to their respective host adapter ID allocation), what steps need to be taken to safeguard integrity of data on hard drives shared by such a method? A: The systems should be using the persistent reserve commands to reserve the drive to insure that the data does not get overwritten. SPC-2 has the reserve commands; it can be found on the SCSI Committee (T10) web site at www.t10.org. Q: I am doing a trade study on various buses available for a high-speed control system. What is the time-to-wire latency for issuing a command? For example, I want to change the state of a communications system from "transmit data" to "receive data." I can toggle To alternate back and forth between two states. toggle - To change a bit from whatever state it is in to the other state; to change from 1 to 0 or from 0 to 1. This comes from "toggle switches", such as standard light switches, though the word "toggle" actually refers to one line to do this. How fast can I toggle that line? A: This is a function of the controllers, not the bus. The bus swaps direction in 400 ns. Arbitration for the bus is 1.6 microseconds. Command to the bus timing is strictly a function of the controller and the arbitration time to get the bus. |
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