SCSI

Many people who use SCSI regularly but are not involved with it on a day-to-day basis have some confusion relative to the "new" LVD SCSI, especially with regard to its compatibility with the "older" differential (HVD) and single-ended (SE) SCSI varieties. I will try to clarify the situation a little. First, however, let me define what is meant by SE, HVD and LVD SCSI. These terms refer to the manner in which the SCSI interface places signals on the SCSI cabling. The original SCSI specifications defined only the single-ended (SE) interface. I have heard this referred to as "regular" SCSI, however, "regular" is not a SCSI term. Anyway, the SE interface drives each signal line against ground. Narrow (8-bit) SCSI has 18 signal lines plus a few others for term power and some reserved pins. SE signals are susceptible to noise and do not offer very long cable lengths. Slow SCSI has a maximum recommended cable length of 6 meters (20 feet); Fast SCSI is 3 meters (10 feet); and Ultra SCSI is 3 meters for 4 or fewer active SCSI IDs but only 1.5 meters (5 feet) for 5 or more active IDs. The introduction of differential SCSI (now called HVD for High Voltage Differential) overcame many of these cable problems. The HVD interface drives two lines for each SCSI signal - one line is the inverse of the other line and the SCSI signal is the difference (hence, the term differential) between these two lines. This results in a more robust signal that is not as susceptible to noise as the SE interface and offers SCSI cable lengths of 25 meters (82 feet) regardless of the speed of the bus. The maximum data throughput of the SE and HVD interface is Ultra SCSI (also called Fast-20) offering 20 Mbytes/sec throughput for narrow SCSI and 40 Mbytes/sec throughput for wide SCSI. The disadvantage of the HVD interface is that it is more expensive than the SE interface. When the members of the X3T10 SCSI Committee looked for ways of again doubling the data throughput of SCSI, they realized that the 5 volt logic used in SE and HVD SCSI would not allow 40 Megatransfers/sec data rates. Enter LVD SCSI. LVD uses 3.3 volt logic and common sense tells us that you can change a signal through a 3.3 volt range faster than through a 5 volt range. Making the interface a differential interface (thus the term Low Voltage Differential) retained the advantages of noise rejection and longer cable lengths that differential signals offer. This lower voltage offers another advantage. The lower voltage and lower current requirements of LVD SCSI drivers means lower heat dissipation. That means that the differential transceivers can be included on the LVD SCSI interface ASIC which results in an interface with a smaller parts count, lower parts cost, a requirement for less real estate on the pcb and increased reliability. Those of us who have been around long enough know that every new variety of SCSI has always been backward compatible. But LVD SCSI is not compatible with the signal levels of previous forms of differential SCSI. Some clever individual or individuals on the X3T10 Committee came up with the idea of "multimode" LVD (called LVD/MSE in the SCSI specs for Multimode Single-Ended) which is an LVD interface that switches from LVD to SE when it is connected to a SE bus. Now the manufacturers of SCSI peripherals need to produce only the one LVD variety of SCSI drive, instead of two varieties, SE and HVD. That saves them money and it is passed on to their customers. By the way, all the LVD hard drives have the wide SCSI interface. You can place these multimode LVD (or LVD/MSE) wide devices on a SE narrow SCSI bus by the use of an adapter to change the 68 pin high density connector on the hard drive to a 50 pin connector for the cable to your SCSI bus. The unused pins on the LVD drive should be terminated. This can cause some heartburn if the LVD drive is not the last device at the end of the SCSI cabling. If it is the last device on the cabling, you can get a 68 pin to 50