XETEC LT. KERNAL HARD DRIVE


For the Commodore 64 & 128 Computers

Written by eBay's 94Bravo





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Note: If you have a working Lt. Kernal system and simply want to Add another drive:

- You'll still need to understand the information cover below and on the "Editing SYSGEN" page
- Edit your SYSGEN disk with new drive parameters, and
- Then, install the modified SYSGEN DOS software


Adding or Replacing Drives:

This section describes the procedure for adding or replacing Hard Drives to the Lt. Kernal system. Also discussed are Drive Enclosures and connecting the Host Adapters' Non-Standard SCSI-1 output to the drive via a special SCSI Adapter assembly. Lastly, it is strongly recommended that you read the information covered in both the SYSGEN and HARD DRIVE sections before modifying your Lt. Kernal because this data will help you understand how to maximize the Kernals' storage capacity.

Many SCSI hard drives will work with the LTK, but not all. We'll describe how the hard drive 'parameters' or 'specs' determine which Drives are compatible and how & why these drive parameters are entered into the SYSGEN DOS software. As explained below, the drive data must be entered into the SYSGEN software in order for the system to properly recognize the drive. Unlike a modern PC or Mac, the Commodore and LTK don't have the ability to interrogate the drive (auto-install) and store the drive parameters by themselves. So, you need to manually enter this data into the SYSGEN software.

Since the original Fiscal and Xetec system design used external hard drive enclosures to house the drive(s), we'll also look at some options you can adapt to your Lt. Kernal system. Other than the drive enclosure containing its own power supply for the drive, there's nothing special about the external case. However, connecting the enclosure to the Host Adapter has raised questions in the past, so we'll cover this issue as well.

As mentioned previously, this series of articles only addresses the Xetec Host Adapter designed for use with embedded-controller SCSI hard drives. Fiscals' (and Xetecs') older and less common designs, which use OMTI controller cards, are not covered in this section.

To pick a SCSI Drive for use with the Lt. Kernal, you'll need to lookup the Drives':

  • Number of HEADS
  • Number of SECTORS/TRACK
  • Number of CYLINDERS
  • Can the drives' Parity be disabled



Selecting a Hard Drives:

Many Lt. Kernal systems were shipped with either a 20 Meg or 40 Meg MiniScribe, SCSI-1, embedded controller Hard Drive. Those MiniScribe 'stepper-motor' hard drives make a distinctive sound during self-test at startup! Image what it must sound like to have seven Miniscribes crammed into an external enclosure?

Lt. Kernal - Add-on Drive

Xetec also offered an ADD-ON drive option. Xetec would send you another external drive case (with drive) and a new/modified original SYSGEN software diskette. Your new drive would be set to SCSI address "1" (to go along with your existing address "0" drive) and the new drive Parameters added to the next sequential drive-table cell position on your new SYSGEN disk. All you needed to do was daisy-chain the new enclosure in your system and reinstall your new software. Of course, your old copy of the original SYSGEN would be useless because it wouldn't know the new drive existed! Needless to say, this software dependence on Xetec probably helped kill the product. But the information in this article will remove your dependency on the non-existing company.

So, what if you don't want to add another Miniscribe Hard Drive? Moreover, why would you want to use this unreliable and slow drive anyway? Xetec used the Miniscribe because they were cheap and common for the 1980s. However, you don't have the restriction of 80's technology and can used thousands of compatible drive now! Besides, the slew rate and transfer rate of the old Miniscribe drives makes them an undesirable drive given the choices you have in newer drives.

As long as the drive is SCSI compliant and has some other common features, you can use a HH 3-1/2" Seagate/Quantum/Sun/Apple/etc. or a FH 5-1/4 Prime/Micropolis/etc. or many other drives. As a matter of fact, if all you have are 1 gigabyte drives, they will work just fine! Whatever drive or drives you pick for the Lt. Kernal, there is a limit to the total megabyte capacity you can use! Xetec said in the Manual that the limit was 140 Meg, but this was mainly do to their use of 20 Meg drives. Just keep in mind that regardless of the Drive capacity, the Lt. Kernal only understands how many Cylinders, Heads and Sectors/Track are programmed/entered into the SYSGEN software. Therefore, if you use a 1 gigabyte hard drive, you'll only program the number of Cylinders, Heads and Sectors/Track that will equal the maximum limit of the Lt. Kernals' capacity.

The Host Adapter is designed to use 'embedded controller' SCSI hard drives. The "N" type (Narrow, 8-Bit) 'embedded' SCSI drive is the most common (cheapest) and easiest to integrate into the Lt. Kernal. While Narrow, 8-bit drives are no longer manufactured, these drives are readily available online for less than $10.00.  Here are some drive parameters to look for when choosing a hard drive for use in the Lt. Kernal:

  • "N" type (embedded) 50-pin SCSI hard drives
  • 1 to 10 Heads
  • Cylinder count from several 100s to 1500
  • 15 to 40 Sectors per Track

As you can see, this includes a Bunch of drives. It really doesn't seem to matter about manufacturer's brand. It also doesn't matter if the drive contains a computer manufacture's Boot ROM (e.g., Sun, Apple, etc.). To start with an example, let's look at a Seagate ST1201N. First, it's an 'embedded' drive and has:

  • 9 HEADS
  • 1068 CYLINDERS and
  • 36 SECTORS/TRACK

Here are some other drive characteristics you must consider when choosing a SCSI drive:

  • Choose drives with selectable jumpers for 'Start Unit' (i.e. drive motor starts without commands)
  • Okay to use Apple (Mac) or Sun computer drives that contain special Boot ROMs.
  • Drives with selectable 'Parity'. Usually, the LTK does NOT like Parity Enabled with small drives!
    • The Host Adapter does Not use the Parity line. Therefore, since the Parity line is 'floating', the drive controller may think there are continuous Parity Errors, unless Parity is Disabled on the Drive Controller.
    • Example, Quantum ELS Prodrives Won't work (no way to disable Parity)
    • Quantum LPS Prodrives Do work (because you can remove Parity jumper - disable)
  • If you choose a variable Sectors/Track drive, ALWAYS use the lowest number
    • Example: Maxtor LXT-213SY (Sun) has 34-56 Sectors/Track (depends on where the Head is located on the Disk).
    • On a PC, default interrogation of the drive shows 42 Sectors/Track (average) which, if used on the LTK, will not work. But, by setting to 34 (lowest value), the same drive works fine.
    • Variable Sectors/Track drives were not available when the Lt. Kernal was designed, so it was never a design consideration and why it is not talked about in the Manual.
  • If all you have are one gigabyte drives, they work great with the LTK
    • No, you won't get a Gig of storage, but the drives are Fast and cheap!
    • You'll have to de-tune the drive (use less Cylinders, Heads, Sectors/Track numbers) when you edit the SYSGEN disk.
    • Just because the drive has more Cylinders/Heads/Sectors, you just don't use them all!
      • The LTK only knows what you program (which is very convenient!)
  • Generally, higher capacity drives have higher slew, seek & data transfer rates
    • Big speed difference compared to a Miniscribe!
  • Whenever possible, check the drive specs online or in a drive 'bible' with the data returned by your PC or Mac formatting program (as above).
  • LAST Drive MUST BE TERMINATED! Follow normal SCSI termination procedures
    • Un-terminated drives will either operate unpredictably or not be seen by the Host Adapter.
    • Yes, if you use 2 or more drives, only Terminate the drive furthest from the Host Adapter.
    • Termination Power must be supplied by the Drive, Not the SCSI Bus!
      • With multiple drives, power the bus on the Last drive.

The Lt. Kernal only needs Three pieces of drive parameter data. The reason for this is that Cylinders (most data you can fetch without moving the Head), Number of Heads and the number of Sectors per Track, defines how much data can be stored on a drive. If you've looked-up the drive data and determined the drive parameters, the Sectors/Track number and number of Heads are numbers directly entered into the SYSGEN software. However, even though the number of Cylinders generally ranges only in the thousands and could be easily entered, the Cylinder number is entered into the SYSGEN software in HI Byte/LOW Byte format.  Why?  Any number greater than 256 won't fit into an 8-bit byte.

HiByte/LoByte isn't all that complicated to understand, but needs to be discussed here. In our Seagate ST1201N example above, this drive has 1068 Cylinders. So, if we divide the number of Cylinders by 256 (HiByte), we come up with 4 (1068/256=4.xxx). Then, if we multiple 256 times 4 and subtract that product from 1068, we get 44 (1068-(256*4)=44) or the LoByte. These two numbers (4 and 44) are the representative cylinder numbers entered into the Kernals' SYSGEN Disk.

Now that you've gone to all the trouble of calculating CSH data, it'll never be directly used by the Lt. Kernal. Other than this data being stored on the hard drive for bootup, LTK DOS uses the drive parameters to convert to Physical Block addressing. Block addressing is used when LK DOS assembles a SCSI Command Descriptor Block in the Commodore which is then passed through the Host Adapter to the drive controller.

When looking for hard drives and calculating drive parameters, keep in mind that the Lt. Kernal can only use a maximum of about 150meg (per drive) of the drives total capacity (at any given time).  The ideal drive size is between 160 Meg and 200 Meg.  However, I prefer to use 1 gig drives because these drives are faster and cheaper, even though I'll waste most of the drive's capacity.  (Is there a way to use more of the drives capacity? - Yes!)  Regardless of the drive capacity, the secret to more Lt. Kernal storage capacity is to use more than one drive!

Whatever the storage, we're talking about an 8-bit Commodore. Commodore programs typically range from 10 to 200 Commodore Blocks in size. That means you can store thousands of Commodore files in a relatively small amount of space. So, whatever hard drive you choose to use with the Lt. Kernal, you can add enough megabyte storage capacity and never run out of drive space!

The bottom line is that it's easy to find a compatible drive that will work with the Lt. Kernal system! Once you selected a drive (or drives) and have determined the drive's number of Cylinders, Heads and Sectors/Track, you're ready to edit the SYSGEN software.







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