ASK THE SCSI EXPERT.

Q: How do I properly terminate a chain that goes from SCSI-2 to SCSI-1? We have a new system with a dual-channel, on-board SCSI controller. One channel is wide, the other is narrow. The wide channel works perfectly with two striped IBM disk drives. Connected to the narrow channel is: first, an internal CD-ROM reader, then a CD-ROM Writer, then the cable goes to a 50-pin, high-density SCSI-2 bulkhead connector. An adapter cable connects to this and goes to a Zip drive (25-pin SCSI-1). Termination is enabled on the Zip drive.

Is this chain properly terminated? Where did half of the connectors go between the SCSI-2 and SCSI-1 connectors? Are they terminated or just floating?

The problem that I'm having with this configuration is that the CD-writer regularly runs into SCSI errors during recording and ruins the blank disc. This occurs even though each item on the bus has its own ID and only the last item is terminated, with the SCSI controller providing its termination.

A: It sounds like you terminated both ends of the 50-pin cable with the controller at one end and the Zip drive at the other end. There are several signal return lines that are all connected to six ground lines.

When writing multiple CDs, typically the first one writes OK, but the second or third CD gets errors. I had a similar situation with my own CD-R and was getting errors until I put an extra fan in to cool it. I found that I could only write one CD at the most; the second always died until I aimed the extra fan at the CD. After that, I never had another occurrence of that problem. The main problem is normal under run errors caused from fragmented disk or too much bus and system activity during the write.

Q: Will an Ultra2 SCSI drive, work on a Wide SCSI (68 pin) or SCSI II (50 pin)? What is Fibre Channel used for and how is it connected?

A: Ultra2 drives are multimode. They will run in LVD SCSI mode at Ultra2 (Fast-40) speeds or single-ended at Ultra (Fast20) speeds. They can even be connected to the 50-pin connector if the upper byte is terminated. The drive is restricted to addresses zero to six; seven is normally the controller. Addresses 8-15 can not be used.

Fibre Channel is a different interface; it is normally used to connect servers to RAID (Redundant Array of Inexpensive Disks) Inside the RAID box is parallel SCSI.

Q: Is it possible to link two computers trough two SCSI cards to form, a simple network between the systems?

A: The operating systems and the controllers are not set up to act as a target; they want to be the initiator. Most adapters will work in target or initiator mode, but the drivers and operating system only support initiator mode. In some of the earlier Apple systems, the laptop acts, as a slave when it is connected to desktop SCSI

Q: Is there such an animal as an Active SCSI-3(i.e., not passive or differential)? Where can I get one? Please also explain exactly what it is and in what application this would be used.

A: These terms generally refer to terminators. The active terminator is required for the higher single-ended speeds such as Fast SCSI and Ultra SCSI. The original terminators, used for SCSI were passive.

Ultra2 SCSI and Ultra160 SCSI use LVD SCSI (Low Voltage Differential); which, in most cases, is multimode. These terminators will run in LVD mode if everything on the bus is LVD or they will switch to single ended active terminators if there are single-ended devices on the bus. Neither singled-ended nor LVD SCSI is the right pin out for the old differential standard. Active SCSI terminators are available at any PC store that sells SCSI devices.