Lt Kernal - Enclosures, SCSI Adapter and Cables

Besides your Hard Drive, there are three other hardware items you'll need to complete your Lt. Kernal system: Drive case, special SCSI adapter assembly and cabling. Drive enclosures and cables are available from many sources, but the SCSI adapter assembly is unique to the Lt. Kernal.

Use or buy any hard drive enclosure! As long as the case includes an internal power supply (+5VDC & +12VDC) that will power the drive or drives, has a cooling fan and power switch, the enclosure will work. I have used Macintosh, Sun, PC towers and other 3rd-party cases and they all have advantages and disadvantages to their use. A PC Tower can house a lot of drives and has a capable power supply, but it's a space hog. However, if you have drive bay space available in your PC, use your PC to house your Lt. Kernal drives too! I've also used Sun's external drive cases and changed the SCSI connectors. The only cases to think twice about are Apple (Mac-II) cases!

It's too bad, but Macintosh-II external hard drive cases could be perfect. They are readily available, have a power supply, fan and power switches. Best of all, they are SCSI-designed cases with 50-pin Centronics data input/out connectors! And, as with most Apple Mac-II computers, Mac drive cases connect to the computer via a DB-25 to 50-Centronic cable (i.e., the LTK Host Adapter has a DB25(f) connector too)! UNFORTUNATELY, if you connect this Mac Case/Cable combo to your Host Adapter, you'll probably Fry the Kernals' Host Adapter!

Note: Pins 14 & 15 on the Host Adapter's DB-25 connector are grounded. These same pins send ground, via cable, to the SCSI Adapter board's DB-25 connector grounding screws. This separate Ground is for cable Shielding.

* Pin 10 is the Host Adapter Enable input. This pin is normally Grounded (Low) via cable to the SCSI common ground on the 50-pin Header connector.

- Pin 10 is Switched by Xetec's Multiplexer unit via Mux U11.
* Pins 11, 12 and 13 are also grounded on the Host Adapter and at the SCSI Adapter board (via cable) to the SCSI common ground on the 50-pin Header connector (ensure common ground between Host Adapter and Hard Drive).

* Pin 16 on Host Adapter J1 is connected to +5VDC on PCB (at RN1 source).

* Regarding "Termination Power", Pins 25 & 26 on the 50-pin SCSI Header connector are Not connected on a Lt. Kernal system (see the SCSI adapter board Artwork in the FILES section). Therefore, ensure that pin 25 (Termination Power) on the Mac DB-25 is Not connected. Termination Power must be supplies by the Drive on a Lt. Kernal system.

There's one SCSI-1 standard (ANSI X3.131-1986). In the early LTK designs, their SASI interface (SCSI's ancestor) to the OMTI controller card is identical to the SCSI-1 standard. However, both Macintosh and the newer Lt. Kernal used an alternative and incorrect cabling method to the drive - each wired differently! So, attempting to use the Mac DB25 cable with the Lt. Kernal will result in NONE of the 9 data and 9 control lines going to the correct pin on the Host Adapter. And, notwithstanding Macintosh, if Xetec would have followed the ANSI standard and used a DB-50 connector (instead of a DB-25) on the Host Adapter, this section of the website would not exist!

As much as the Mac case/cable combo looks good, you would have to modify the internal (ribbon) wiring to the drive so much that it's more trouble than it's worth. Therefore, the simplest way to deal with connecting the Host Adapter to a hard drive is to do it the same way it was done by Xetec - using a specially designed SCSI interface assembly and standard cables. Your only other option is to build a DB-25 to DB-25 gender changer (like a Null modem) using the above chart for wiring. By building this 'DB-25 Adapter', you will then be able to use Mac SCSI cables and drive enclosures.

Xetec used three components to connect the Host Adapter to a hard drive: (A) a DB25(m) to DB25(m) cable, (B) a DB25(f) to 50-pin Header(m) SCSI adapter board and (C) 50-pin header socket ribbon cable. The DB25 cable is cheap and only needs to be a pin-to-pin match (i.e., all pins connect to the same number on the other end as used with switch boxes, extensions, etc.). The special SCSI adapter assembly consists of a DB25 pc-mounted, 90-degree connector (which can be chassis mounted in the drive case) and a 50-pin male Header connector, both mounted on a small single-sided circuit board with appropriate (and unique) connections between the two connectors.

The DB25 cable and special SCSI adapter board turns out to be the most expedient means of connecting the Host Adapter to the hard drive. The way the special adapter is designed also makes it easy to install in the enclosure. If you do use a Sun or Mac case (or any case that does not use a DB25 SCSI connector), you'll need to fabricate a mounting plate or adapt an existing bracket (e.g., Jameco # 101733CP). Also, if you decide to use a PC case to mount your drives, the Jameco bracket is the easiest to modify for mounting the SCSI adapter in the case.