For the Commodore 64 & 128 Computers

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HOST ADAPTER - Details & Troubleshooting

Xetec HOST Adapter:

This section describes the Host Adapter hardware subsystems and provides recommendations for component replacement or substitution. The sections on System Firmware and RAM also partially explain the operational theory of how the system works. Topics covered include:


The Host Adapter was designed around a Peripheral Interface Adapter (PIA), MC68B21P. This was a commonly used device and is still available today. The "B" indicates this is a 2 MHz device that was necessary for Commodore 128 compatibility. There is also a 68A21 (1.5 MHz) and 6821 (1 MHz) version of this device. If you are using the Lt. Kernal on a C64, you could use any of the three versions as a replacement component, but it's recommended to simply use the commonly available 68B21P device.

The PIA provides the means of interfacing the Commodore bus and hard drive controller via two 8-bit data buses. The PIA's Control Register and two Data Direction Registers (DDR) control the two-way flow and are switched by LK DOS and four control lines.  However, when the Commodore is first turned on, *Reset initializes the PIA which zeros all PIA registers, sets the PA & PB data bus as Inputs and disables all interrupts.  So, how does the PIA know what to do next?  The hardwired code contained in PLD-1, PLD-2 and the Boot firmware contained in U7 EPROM set the PIA registers and start communications between the Commodore and hard drive.  Once Bootup is complete, LTK DOS takes over control of the PIA.

When the Commodore needs to send a command to the drive, LK DOS assembles a SCSI Command Descriptor Block (CDB) command.  This CDB is then loaded into one of the PIA data registers and sets the DDR to output this CDB to the hard drive controller. The hard drive controller interprets the CDB and responds. If, for example, the CDB requested a particular block of data to be read from the drive, the PIA needs to change the direction of data flow (via DDR) and wait for the drive to respond. Until the data is ready, the Commodore does not sit idol; it goes on with what it needs to do. In the mean time, when the hard drive controller sends the block of data, the data passes through the PIA and goes into part of lower Shadow RAM. When loaded into RAM, the Host Adapter sends an interrupt to let the Commodore know the data is ready. This is the concept of how data and handshaking takes place between the Commodore and hard drive controller.

Program Logic Devices (PLD):

The logic contained in the two CMOS, 20-pin DIP, P5C032 PLD (Programmable Logic Device) chips (U1 and U2) provide the decoding and control logic needed to map the Host Adapter Shadow RAM in and out of processor space. The 5C032's are UV-erasable, 1st-generation Intel PLDs and are capable of Combinatorial or Register logic (combinatorial in the LTK).  As mentioned before, the 'Security Bit' was set when these devices were programmed by Xetec in an attempted to protect their LTK design.  However, because an EPROM is called a poor mans PLD (and the logic is supposed to be combinatorial), decoding the PLD's logic is possible.  You need to know that all inputs (pins 1 - 9 plus pin 11) are used as inputs and pin 14 of U2 plus pin 15 of U1 are wired-OR (*NMI) Input/Outputs (I.C.Q.U.B.).; - Pin 14 & 19 of U1 and pin 13 of U2 may be unused inputs, an I/O wraparound or N/C; - Pin 13 of U1 (CAEC) is an Input/Output; - based upon U3, pin 23 (*CS2), Pin 12 of U2 may be an Output only or an Input/Output  (see datasheet).

Once you have decoded the logic table, there's no need to program another rare P5C032 blank.  Instead, use a direct substitute (e.g., EP320IPC) or adapt any number of similar devices including a GAL (16V8) or PEEL (18CV8P-25). No, you can't use the logic table directly, but there are any number of programs you can use for conversion (including PLDShell, ABEL, etc.).

System Firmware:

U7 of the Host Adapter contains Bootup Firmware. This not-very-populated 2764 EPROM performs some very important tasks during startup. When the Computer is first turned on, the Host Adapter's PIA and 16K RAM need to be configured and LK DOS needs to downloaded from the hard drive. (The firmware contained in the EPROM is slit into two sections - first half is for C64 bootup while the second half is for C128.  Both serve the identical function).  So, if we looked at the first nine (9) bytes in the EPROM, we begin to see how this initialization is done.  The following is a simplified and compressed explanation of the Lt. Kernal bootup sequence. Remember that once the EPROM has booted the Lt. Kernal, it is no longer used.

The fifth to ninth bytes of the EPROM contain the familiar "CBM80"! Well, what do you know, the EPROM contains an AutoBoot program (Commodores' normal cartridge autoload routine). Because the Host Adapter is plugged into the Expansion Port, byte zero of the EPROM is also looking at $8000 (hex) of the Commodore! So when power is first applied to the computer, the Commodore looks at bytes 5 through 9 of the EPROM and if it finds "CBM80", it continues with an AutoBoot routine. Startup then continues by looking at the first 4 bytes of the EPROM ($8000-$8003) to determine the Cold and Warm vectors, both of which turn out be $8014. From now on, it's easy to see that bootup is just a normal Cartridge bootup routine. Remember that at this point in time, there is No Lt. Kernal or Lt. Kernal DOS software at work; only firmware programs contained in the Host Adapter's EPROM.  This same bootup happens for the C128. The Host Adapter tricks the computer to look in the second half of the EPROM, thinking it is $8000 (Host Adapter P1, pins 1&2 either grounds or is High at pin 2 of U7 - bank switches either 32K halves of the EPROM).

The program at $8014 does several things. It sets the PIA control registers, assembles a CDB and sends the CDB through the Host Adapter to repeatedly select SCSI ID 0. If there is no response from the hard drive, LTK bootup is aborted and the Commodore boots to its normal startup screen. But if the computer determines that the responding device is a hard drive, the drive is instructed to send the first Block of data from cylinder 0, track 0, sector 0 of LUN 0. (Note: In a modern SCSI computer system, all SCSIs IDs are polled, but only SCSI ID 0 is addressed in the Lt. Kernal). If the hard drive responds, the data from this read should contain three pieces of data; the SYSGEN-installed Serial Number, hard drive parameters for ALL installed hard drives and rudimentary code needed to load the balance of the LK DOS. This data block is temporarily stored in the lower 8K of Shadow RAM.

NOTE:  Gee, I wonder, is there enough space left in the EPROM to first send a "Disable Parity" command to the Drive?  Sure would be nice to use a 2.5 inch laptop drive :)

If the serial number data from the hard drive read does not match the serial number stored in the EPROM (bytes $0A to $11) the whole bootup process comes to a screeching halt! If the serial numbers do match, the supplemental boot code and the hard drive table data (now in Shadow RAM) are used to read LK DOS from the hard drive as part of the EPROM bootup process. Once the serial numbers match and the LK DOS is loaded from the hard drive, the Lt. Kernal is now running under LK DOS and the boot EPROM is subsequently switched out as bootup is complete.

If you want to know how the hard drive parameters got onto the drive in the first place, refer to the SYSGEN and Hard Drive sections. The concept of the AutoBoot routine is straight forward, but unless you understand what's supposed to happen, you will have difficulties diagnosing the system.

This is also why a proper SYSGEN installation onto the hard drive is important and why some data loaded during a SYSGEN needs to 'match' the Firmware stored in the EPROM. See the sections on SYSGEN and Hard Drives for more information on how to verify/change the serial number on the SYSGEN disk.

Of course, when you install a new hard drive, there is No DOS data to load! So, refer to the Hard Drive and SYSGEN sections to see how this is handled.

16K Shadow RAM:

There are two CXK5864PN-15L 8K RAM chips in the Host Adapter (U8 and U9). Some Host Adapters used Dallas RA000001A RAM chips. In either case, these are the same as a more common 6264P RAM chip. I guess the first question is why does the Host Adapter have RAM at all? The Commodore has RAM and a Kernal ROM and the Host Adapter has an EPROM.

The Host Adapter has extra RAM because the Commodore itself was not designed with a Disk Operating System, or DOS. The only thing the Commodore has is a kind of BIOS (Basic Input/Output System) or startup routines contained in the Kernal ROM. If the LK DOS was loaded into Commodore RAM (as it was in the earlier LTK design), there won't be as much RAM left for application software. Also, in the early LTK designs when LK DOS was loaded in Commodore RAM under the Kernal ROM, it lead to software conflicts with other programs. And, because of these early software conflicts, Fiscal decided that No LK DOS software should reside in the Commodore RAM, resulting in Shadow RAM being incorporated into the Host Adapter's design.

So, by having two extra 8K RAM chips in the Host Adapter, you can start to see why and how they're used. Back when the Host Adapter's firmware was beginning an initial boot, shadow RAM is switch off. But after the first Block of data from cylinder 0, track 0, sector 0 of LUN 0 was read, it's stored in part of lower 8K RAM. The Bootup firmware then (1) sends new vectors to lower ram in the Commodore, (2) the Commodore Kernal ROM is copied into the upper 8K RAM and then (3) this copy of Kernal ROM (in upper 8K RAM) is modified (vector jumps) by Host Adapter firmware.

Now that the upper 8K RAM contains a modified Kernal ROM, it's treated like a ROM from then on (Write disabled). The lower 8K RAM (which already contains the serial number, drive parameters, LTK LU number and hard bootstrap code) is then loaded with needed LK DOS mini-subroutine code from the hard drive. It would have been impossible to load all LK DOS into Shadow RAM, which created some problems for programmers. So what we have is:

  • A LTK-modified Kernal ROM in upper 8K RAM, acting like a ROM
  • Serial number and all installed hard drive parameters loaded in part of lower 8K RAM
  • Some LK DOS code also loaded in the Lower 8K RAM
  • Two 512 byte blocks (in lower RAM) are set aside for input from and output to the hard drive
  • Host Adapter is looking at RAM in the Commodore at $8000 (yes, both C64 and C128)
  • Shadow RAM is then Output Disabled
  • And, because the modified Kernal ROM and LK DOS are hiding in Host Adapter's RAM, that's why it's referred to as SHADOW RAM

From this point forward, Shadow RAM is primarily used as a temporary holding place for Data exchanges between the hard drive and Commodore. One trick feature is that Data from the hard drive is in 512 byte blocks, which needs to be split into Commodore 256 byte blocks.  The same conversion takes place in reverse (Commodore to hard drive) by stacking and splitting data in Shadow RAM.  (On a personal note, John and Marty Flickinger were two software guys who needed a serious vacation after this design! :)

Needless to say, if you have a bad RAM chip in the Host Adapter, you can expect to see some mind-altering symptoms and can waste time troubleshooting. If you suspect you've got a bad RAM chip, switching out these $2 parts is better than countless hours spent stirring at an oscilloscope screen. (Yes, a scope will tell you if one of the two RAM chips have blown output chip drivers and/or lead to why the outputs are being pulled low.)

Note: All of the other active components used in the Host Adapter are commonly available '74LS' series TTL integrated circuits with the exception of one 2N3904 transistor. (See the Files page for Datasheets on all integrated circuits used in the Host Adapter)  Finally and if you have the desire, reverse-engineering the Lt. Kernal is one of the best reviews of 1980's, 8-bit world, designs!

Host Adapter Troubleshooting Tips:

*** First, start with the basics! Turn everything off. Disconnect the LTK Host Adapter and then power-up the Commodore by itself to ensure you have a working computer. And, as with ANY power-consuming device connected to the Expansion Port, consider if you are using a stock/wimpy power supply or a more reliable after-market or custom-built power source? Are the +5 volt DC and 9 volt AC levels correct? Can you cook an egg on the power supply (i.e., stable outputs)? Power supplies that are marginally operating will generally seem to work fine when the computer is first turned on. However, after a minute or so, if the system seems to be performing erratically, try another power supply.

*** If the power supply is operating properly, but you only see a partial Lt. Kernal startup screen, which then returns to a normal Commodore startup screen, the problem is most likely a corrupted LK DOS software on the hard drive. Reinstalling LK DOS software should remedy this problem, but (by reinstalling LK DOS software) you will destroy all files on LU 10 (DOS) including any user files you placed there.  However, All of your files in LU 0 through LU 9 (if you used that many LUs) are still on the drive and are accessible again after you have reinstalled DOS and Configured the system back to your previous LU settings.

*** If the Commodore works normally by itself, turn it off again and ensure that the two Control lines are still properly connected inside the Commodore 64. If you're using a C128, ensure that the daughter-board is properly seated into U7 IC socket and HIRAM is connected to Pin 23 of U11. After checking the Host Adapter edge-connector for corrosion, reconnect the Host Adapter in the Expansion Port, ensuring that it's properly seated in the connector. Note: When inserting the Host Adapter into the Expansion port, Do Not rock the Host back-and-forth! In worn Expansion port connectors, it is possible to force the pins left and right, which could short to adjacent card-edge pads. Also make sure that the HIRAM/CAEC (C64) or IDE cable (C128) are connected to the correct place on the Host Adapter.

*** Are you using a C128 or C128D AND did you just install a new (different) hard drive? If so, the Lt. Kernal Defaults to 40-Column mode (not 80-column like the C128). So, change the C128 to 40-column mode. Now that you can bootup, install LK DOS software and do a full reset (remember to switch to 40-column mode on the C128). Next, go to the Config screen, select F3 and select C128, 80-column mode for your normal startup. From now on, you'll properly bootup in 80-column mode on your C128.

*** Next, power-up the external hard drive and listen for normal drive spin-up. Then, turn on the Commodore. If the hard drive needs a SCSI Command to spin-up (Start Unit), you've picked the wrong drive for use with the Lt. Kernal system. (Note: Even though 'Start Unit' was one of the optional commands in the original SCSI-1 standard, is it not used by either the Commodore or LK DOS software). If you are installing a new drive for the first time, does the drive have a jumper for 'Start Unit' and is it properly configured? (check the drive specifications either online or in a drive manual)

*** As soon as the Commodore is turned on, you should hear (see) the hard drive being accessed by the Host Adapter (looking for LK DOS software). If the drive starts searching for data, the Host Adapter is functioning as it should and the problem may be either be a bad hard drive, corrupted LK DOS data on the drive or you selected the wrong drive to use with the Kernal (see below and the section of Hard Drives). Some of us keep a spare hard drive with a clean-installed LK DOS for just this testing purpose (i.e., does the spare hard drive boot up normally?) Remember too that older SCSI drives generally die at startup (e.g., old Quantums and 'sticktion').

*** One simple test to determine if the Commodore is talking to the hard drive is to use a drive activity LED connected to the drive. Contrary to what the manual says, (if you are installing LK DOS software for the first time), leave the data cable connected between the Host Adapter & Drive and then apply power to the drive and computer. What you should see is the drive activity LED flash at about a one-second rate. This indicates that the Commodore is correctly trying to access data on the hard drive even though hard drive parameters have not been downloaded into the Host Adapter's Shadow RAM.

Selecting the right hard drive has a lot to do with how the Host Adapter behaves. If the hard drive you are using is one that has been connected to the system and was working before, you might consider simply reinstalling LK DOS software; with or without first formatting the drive (i.e., remap the drives' media for defects). However, if you are trying to use a new or different drive, first read the Hard Drive section.

*** If you are installing a new/different hard drive, did you forget to set it to SCSI ID# 0 (Zero)? The system is hardwired to only look for LK DOS data on SCSI ID#0. Therefore, if the hard drive is set to anything other than ID#0, it will seems as though the Host Adapter is not working.

*** Also, when trying to use a different hard drive, unless the drive is properly Terminated, nothing will work right! It's been my experience that if termination resistors are not installed on a single-drive LTK setup, the Commodore won't even talk to the drive. The drive must be terminated and Termination Power must be supplied by the drive. Older SCSI drives generally use two or three inline 220/330 ohm resistor packs, which are powered by the drive via jumper pins.

*** If the drive you have chosen is set to 'Enable Parity', the Host Adapter will probably not respond. Even though the Parity Line is not connected on the Host Adapter, a 'floating' parity line might be interpreted by the hard drive controller as a 'parity error'. Therefore, the hard drive must have Parity disabled (check the jumpers on the drive). Higher capacity drives, with more sophisticated onboard controllers, seem to be less affected by the drives' Parity settings.

*** If you are using a C128, and switch to '64 mode', did you forget to change from 2 megahertz Fast Mode to 1 MHz (check the F3 screen in Config)? If so, you will only see a blank screen until you change to 1 MHz mode!. Also, an alternative testing approach for a C128 is to disconnect the 10-pin IDE cable from the Host Adapter and connect HIRAM and CAEC lines to a space C64 computer and then retest (see Host Adapter installation).

*** Another comment about using a C128 and a 1571 drive in Fast Serial Mode. When Xetec shipped LTK systems, these systems did not support Fast Serial mode! What Xetec did was to ship an extra serial cable for the 1571 which had the 'SRQ' line cut (disables Fast mode). Later, a modification was devised that required making a change to the C128 circuit board, which does enable Fast Serial mode (see Files section - 'ltkernal.zip').

*** To help you better understand how the Lt. Kernals' Host Adapter works, refer to the Site section on the Multiplexer unit.  The Mux section includes a schematic, timing diagram and detailed troubleshooting procedure that will also give you a good idea how the Host Adapter works.

*** If you power-up the hard drive and then turn on the Commodore and don't hear/see the drive being accessed, wait! If the Host Adapter can't bootup properly from the EPROM, the Commodore should time-out in about 60 seconds and return to a normal C64/C128 startup screen. If this time-out does Not happen, the Host Adapter is probably hung-up in a bootup sequence. The next thing to try would be to replace the EPROM with a spare and test again. This is actually a more common fix than you might think. If replacing the EPROM doesn't resolve the problem, you're left with component troubleshooting.

Following the above suggestions and reading the sections on SYSGEN and Hard Drives will resolve the majority of Kernal problems. The few remaining failure situations require troubleshooting the Host Adapter components. Using an oscilloscope, look to see if the PIA is cycling or is it hung up? What is the level of the reset line? What happens to the R/W lines on RAM at power-up? What are the 'levels' of static Hi and Low outputs/inputs? If you need more help, refer to the Datasheets in the Files section. (The most common failure mode of TTL is a blown output driver)  With the exception of the PLDs, any competent Technician should be able to find a defective Host Adapter TTL component in a matter of minutes. Remember that the TTL circuitry in the Host Adapter is pretty hearty and will survive a lot of abuse. What the Host Adapter doesn't like is being ripped out of the Expansion Port sideways with power applied or directly connecting the Host Adapter to a Mac or PC SCSI bus!

*** I received a defective Host Adapter that was traded for several years on eBay.  A detailed failure analysis showed that it was clear the Host Adapter had been plugged into a Mac or PC SCSI Bus.  After replacing U3, U4 and U6 (the 3 ICs that directly interface with the SCSI bus), the Host Adapter worked fine and is now one of my working spares.

Preventative measures: One additional thing to think about when using the Lt. Kernal is that the Host Adapter plugs into the Commodore's Expansion Port. Therefore, you should ensure that the computer setup and routing of the SCSI data cable (from Host Adapter to Drive) minimizes the chance of pulling the Host Adapter out of the Expansion Port, especially when powered-up! This may seem obvious, but frying the Host Adapter will definitely ruin the Host Adapter and your Day!

Don't forget that the Lt. Kernal works on an SX-64 computer, which means the Host Adapter will be sticking out of the Top of the SX! Therefore, if you really want to use a Lt. Kernal on a SX-64, it is recommended that you modify the SX by mounting the Host Adapter inside the SX-64.

Also, if you plan on using a Port Expander unit for the Host Adapter, select an Expander (e.g. CMD's EX2+1) that has a horizontal connector so that the Host Adapter can sit flat on the desk. If you're thinking about inserting the Host Adapter into a Vertical connector, DON'T! I've personally seen three Host Adapter with edge connectors that were snapped off!

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