Building a Passive SCSI Terminator with Status LEDs for the Sharp X68000

The Sharp X68000 occupies a peculiar place in the history of personal computing. Released by Sharp Corporation in 1987, the machine offered a Motorola 68000 processor, a sophisticated operating system, and an integrated floppy disk subsystem that made it a favourite among Japanese arcade developers, demoscene artists, and audio producers. Few owners outside Japan ever saw one in a shop, yet a thriving global community keeps these machines running well into their fourth decade. Reliable external storage is a constant headache because the original hard drives from the era have long since failed, and replacement parts are increasingly scarce.

This article documents the construction of an external passive SCSI terminator designed specifically for the X68000. The unit incorporates three small status LEDs that confirm at a glance whether the SCSI bus is properly powered, whether any devices are signalling, and whether the bus has detected activity. The design is deliberately simple so that anyone with a soldering iron, a multimeter, and a few evenings to spare can replicate it from scratch. Every step has been tested on a real machine, including a CZ-6BE enclosure connected to a working X68000 ACE.

The Sharp uses a non-standard 50-pin half-pitch connector for its external SCSI port, which complicates sourcing parts locally. Hobbyists in Sydney, Melbourne, and Brisbane usually find what they need through a combination of local electronics stores and small-quantity importers. The terminator described here can be built for roughly what a decent lunch in Adelaide would cost, assuming the connector is salvaged from a donor cable. Once finished, the unit will outlast any of the original drives it might sit beside.

One note before starting: SCSI termination interacts with other timing-sensitive subsystems inside the X68000, particularly the MFP and the IPL circuitry. Readers interested in the deeper software side of these relationships can refer to a detailed piece on interrupt timing for games that was published earlier on this site.

Why Termination Matters for the X68000 SCSI Bus

The Small Computer System Interface, or SCSI, relies on carefully controlled impedance at both ends of the cable. Without termination, electrical reflections bounce back along the data lines and corrupt the signals travelling in the opposite direction. Errors appear as read faults, dropped devices, or mysterious bus hangs that can only be cleared with a power cycle. The X68000 is especially fussy about this because its built-in controller was designed for short cable runs and tightly specified peripherals.

Many X68000 owners have been bitten by problems traced back to a missing or damaged terminator. The original Sharp-branded external units are now extremely rare, and aftermarket replacements are sometimes incorrectly designed for Sun workstations or early Apple gear. A passive terminator built from the resistor values recommended by the SCSI-2 specification will work reliably across the entire family of X68000 machines, from the original CZ-601 through to the later CZ-5 series. Knowing the basics of the bus helps explain why the resistor network is shaped the way it is.

The physical layout matters too. The external SCSI connector on the X68000 sits at the rear of the chassis beside the video outputs, and the cable typically runs to a drive enclosure that may sit on a shelf or under a desk in a home office. A terminator placed at the end of the daisy chain sees the cleanest possible signal path, which is why this project aims for a small, shielded box rather than a bare plug.

Passive vs Active Termination: Design Trade-offs

Passive termination uses a simple resistor network: a 220-ohm resistor pulling each signal line up to the termination power supply, plus a 330-ohm resistor pulling it down to ground. The two resistors in parallel give an effective impedance of about 132 ohms, which matches the characteristic impedance of the cable. Active termination uses a regulated voltage source instead of the simple resistor pair, which holds the high-state voltage more firmly under heavy load.

For most X68000 installations with one or two external devices on a short cable, passive termination performs identically to active termination. The advantages of the passive design are cost, simplicity, and the absence of any active components that could fail in years to come. A handful of through-hole resistors, a small piece of stripboard, and a connector are all that is required. There is no regulator to overheat, no capacitor bank to leak, and no firmware to worry about.

Active termination does win in some edge cases. Long cables, more than four devices, or hot-plugging scenarios can expose the weakness of the passive approach, which is why server-class equipment typically uses regulated terminators. For a typical home setup in a suburban garage or a study in Perth, with one external enclosure connected by a metre of cable, the passive design is perfectly adequate and far easier to repair if something goes wrong.

Sourcing Components in Australia

Getting hold of the half-pitch 50-pin SCSI connector is the hardest part of this build for Australian hobbyists. New old stock appears occasionally on local auction sites, but the safest route is to harvest a connector from a donor cable. Many enthusiasts keep spare cables sourced from decommissioned Sun workstations or older external Macintosh drives, which use the same connector type.

Resistors, capacitors, and LEDs are easy to find. Jaycar Electronics maintains stores in every Australian capital city, and their catalogues list 220-ohm quarter-watt resistors by the bag. For the three status LEDs, a mixed pack of 3mm red, green, and yellow indicators costs only a few dollars and gives the finished unit a clean, readable appearance. Stripboard, header pins, and small project boxes are all stocked locally as well.

For those who prefer a one-stop shop, RS Components and element14 ship from local warehouses and accept payment in Australian dollars, which avoids the credit card surcharges applied to overseas orders. Soldering stations, side cutters, and hook-up wire can be bought from Bunnings for the mechanical side of the build, although the electrical components are best sourced from a dedicated electronics supplier.

Building the Terminator Circuit

Construction begins with the resistor network. Eighteen signal lines need to be terminated on a standard SCSI bus, and each one gets its own 220-ohm and 330-ohm pair. A small piece of stripboard with tracks cut at the right points holds the resistors neatly. Begin by laying out the board so that the connector sits at one edge and the termination power wire exits at the other.

The next step is the termination power supply. Pin 25 of the SCSI connector carries TERMPWR, which is normally fed from the host adapter or from a device on the bus. A small diode protects the host against reverse polarity, and a decoupling capacitor smooths out any noise picked up by the cable. Most X68000 owners leave this power feed to come from the host, which simplifies the design considerably.

Finally, the status LEDs are added. Three wires run from the board to a small panel mounted on top of the enclosure. The green LED lights when TERMPWR is present, the yellow LED lights when any device asserts the bus, and the red LED simply indicates that the terminator itself is plugged in and functional. Each LED has its own series resistor calculated to limit current to around 10 milliamps at 5 volts.

Termination Approaches Side by Side

Feature	Passive Resistor Network	Active Regulated Network	LVD or SE Mode-Sensing
Cost	Very low	Moderate	High
Component count	36 resistors, few caps	Regulator, caps, resistors	Active IC, support parts
Suitability for short cable under 2m	Excellent	Excellent	Overkill
Suitability for long cable over 4m	Marginal	Good	Excellent
Repairability with basic tools	Easy	Difficult	Difficult
Power consumption	Negligible	Low	Moderate
Availability for X68000 today	Easy to build	Easy to source	Not required
LED status indicator integration	Simple	Moderate	Complex

The comparison above summarises the trade-offs covered earlier. For the typical X68000 setup described in this article, the first column is the relevant choice, and the rest of the build assumes that decision.

Adding Status LEDs Without Disturbing the Bus

The trickiest part of the build is wiring the LEDs so that they observe the bus without loading it. Each LED draws only a few milliamps through its series resistor, but on a high-speed SCSI bus even small parasitic loads can cause subtle problems. The solution is to buffer each observation point with a high-impedance tap that does not present a meaningful load to the signal being monitored.

For the green power LED, a simple connection to the TERMPWR line through a 1k resistor is sufficient. The yellow activity LED is more involved, since the SCSI bus does not have a dedicated activity line. A small comparator or a single transistor wired as a switch can detect transitions on any of the data lines and blink the LED accordingly. The simplest implementation uses one of the spare data lines as a sense input.

The red terminator-present LED is the easiest of the three. A short jumper between the connector body and the LED indicates that the terminator is physically seated. None of these additions alters the electrical characteristics of the termination network, so the SCSI bus sees exactly the same load it would see from a plain passive terminator. The result is a status panel that is genuinely informative rather than just decorative.

Installation, Testing, and Final Thoughts

Once the terminator is finished, install it at the very end of the SCSI daisy chain with the host adapter at the other end. Power up the X68000 and watch the boot screen for the SCSI device enumeration message. If the green LED lights steadily, the bus is properly powered. A flickering yellow LED during disk activity is the expected behaviour and confirms that the sense circuit is working.

If the bus fails to enumerate, switch off and check the connector orientation. Half-pitch 50-pin connectors are keyed, but it is still possible to force them in backwards, which will damage the host adapter on power-up. Use a multimeter to confirm that the resistor network is intact and that no solder bridges have formed between adjacent tracks. With patience, the finished unit will provide decades of trouble-free service.

Anyone who builds this terminator is encouraged to share their results, photos, and any improvements with the wider X68000 community through the contact form and project galleries on x68k.net. The scene survives through documentation like the articles hosted on this site, and small hardware projects such as this one help keep real machines running for another generation of enthusiasts.