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Macintosh Plus BlueSCSI Installation

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Macintosh Plus logic board CR1 behind SCSI connector (Recap-a-Mac)
The DB-25 SCSI connector J3 and NCR 5380 controller U6A on a Macintosh Plus logic board. The empty diode position CR1, below the left end of J3 beside R3 and C47, is where the termination power diode is fitted. Photo: Recap-a-Mac, cropped.

The Macintosh Plus has no internal hard disk; its only SCSI connection is the DB-25 port J3 on the back of the logic board.[1][2] BlueSCSI, an open-hardware board that presents disk image files on a microSD card to the Mac as SCSI drives, is a documented replacement for a failed or missing external SCSI hard disk.[3] It works on the Plus with two extra requirements: a configuration file that selects BlueSCSI's Mac Plus mode, and a source of power, because the Plus does not supply SCSI termination power.[1][4]

Apple left termination power off the Plus "because of power supply limitations": pin 25 of the Plus SCSI connector, TPWR on other Macs, is not connected.[5] The logic board schematic still shows the circuit. A diode, CR1 (1N4001), is drawn from the +5 V supply to J3 pin 25, and the same drawing lists CR1 as "not used".[2] Fitting that diode is a well-known owner modification that lets a BlueSCSI run from the Mac with no separate power lead.[6][7]

Symptoms

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  • A BlueSCSI plugged into the Plus shows no lights, or lights but the Mac will not start from it.[7]
  • The Mac shows a disk icon with a flashing question mark: it has found no bootable system on the disk image. An image made by or for a Mac emulator often lacks the SCSI driver a real Mac needs.[4]
  • A Plus with the early ROMs 342-0341-A or -B (ROM HI) and 342-0342-A (ROM LO) hangs at start-up if a SCSI device is connected but switched off. Apple's instruction is to switch the SCSI device on before the Mac.[8] See Macintosh Plus Troubleshooting#SCSI.
  • The LED on the BlueSCSI's Raspberry Pi Pico board flashing five times means it cannot see the SD card; three flashes means it found no valid disk images.[4]

Safety

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The USB-powered method needs no tools and the Mac stays closed.[4]

Warning: fitting the diode means taking the logic board out, so the cover comes off and the CRT must be discharged first. Apple warns that the machine contains high voltage and a high-vacuum picture tube, and says never to use a grounding wrist strap until after the CRT has been discharged.[9] See CRT Discharge Procedure. Apple also warns that the edges of the metal chassis may be sharp.[10]

Tools and parts

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  • A BlueSCSI v2 made for a DB-25 port. The DB-25 model's board is only as wide as the connector, and its termination is always on.[11]
  • A microSD card, Class 10 or better, formatted exFAT (preferred, allows larger images) or FAT32.[3][4]
  • For USB power: a USB power adapter rated at least 1 A and a micro USB cable for the Pico board.[4]
  • For the diode: a 1N4001 (50 V, 1 A), which is the part on Apple's schematic, or another 1 A diode with a similar or higher voltage rating.[2][6] Dan Schnur used a 1N5819 Schottky diode for its lower forward voltage drop.[7]
  • For opening the case: a Torx driver long enough to reach the screws, the case pull-apart tool, and a CRT discharge tool.[10][9] Soldering iron, solder and side cutters.

Procedure

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Prepare the SD card

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  1. Format the microSD card exFAT or FAT32. If the card later gives trouble, BlueSCSI recommends reformatting with the SD Memory Card Formatter tool using its "Overwrite format" option.[3][4]
  2. Create the configuration file. At ini.bluescsi.com, click the Mac Plus preset and download the file, or write a plain text file named bluescsi.ini containing the two lines below. MacPlus is BlueSCSI's preset for "Apple Macintosh Plus and similar" and sets its quirks and timing for that machine.[12][13][7]
    [SCSI]

System = MacPlus

  1. Copy bluescsi.ini to the root of the card. On Windows, check that it has not been saved as bluescsi.ini.txt with the extension hidden.[1][4] BlueSCSI's debug logging does not work with System=MacPlus.[1]
  2. Copy a disk image to the root of the card. The file name sets the device: HD for a hard disk, then the SCSI ID (0 to 7, 1 if left out), the LUN, an optional sector size, and .hda as the extension. Text after the sector size is ignored, so HD10_512.hda and HD50_512 System 6.0.8L LC.hda are both valid. Names are limited to 64 characters.[3]
  3. Use a ready-made image from the BlueSCSI Images page, or one made with BlueSCSI's Disk Jockey tool. An image Schnur first tried would not boot his Plus; an image he found for the Plus in the BlueSCSI image library, renamed HD0_512-1GB-MacPlus-6+7.hda, booted at once.[7][4]
  4. Make sure at least one image file is on the card before the BlueSCSI is first powered. With no image files it runs in raw mode, offering the whole card as one disk, and the Mac may then format the card so that a modern computer can no longer read it.[3]

Power option A: USB

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  1. With the Mac switched off, plug the BlueSCSI into the DB-25 SCSI port.[1]
  2. Connect a 1 A USB power adapter to the USB port on the Pico board.[4]
  3. Power the BlueSCSI before switching the Mac on. This is Apple's rule for any SCSI device on a Plus with the early ROMs listed above.[8]

Power option B: fit the CR1 diode

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Macintosh Plus schematic SCSI port J3 and CR1 (Apple 050-0159-A)
Apple's schematic 050-0159-A: the 5380 SCSI controller U6A, connector J3, and CR1 (1N4001) drawn from +5VF to J3 pin 25, TPWR. Anode to +5VF, cathode (bar) to pin 25. Apple Computer.

Read this first. On the Macs that do have TPWR, Apple provides the line "for powering termination resistors only, not for powering SCSI devices", and it left the line off the Plus because of power supply limitations.[5] The diode makes the Plus's own +5 V supply power the BlueSCSI. Owners who have fitted it report it working.[6][7][14] One owner on TinkerDifferent reported weak speaker sound and then a machine that would not start some boots after fitting a 1N4007 in this position. Other members had no trouble with the same modification, and the thread did not find the cause.[14] If the analog board's +5 V output is already marginal, put it right first (Macintosh Plus Low Voltage Repair) or use USB power.

  1. Switch off and unplug the Mac. Remove the battery compartment cover, then the reset/interrupt switch if one is fitted.[10]
  2. Remove the case screws, including the one in the battery compartment, and gently pry off the cover with the pull-apart tool.[10]
  3. Discharge the CRT. See CRT Discharge Procedure.[9]
Macintosh Plus logic board cable and floppy drive cable (Apple Service Source)
The logic board cable and the floppy drive cable, disconnected before the logic board comes out. Apple Computer, Service Source.
  1. Disconnect the floppy drive cable and the logic board cable from the logic board.[9]
Macintosh Plus logic board removal (Apple Service Source)
Sliding the logic board up out of its rails. Apple Computer, Service Source.
  1. Pull back on the top of the logic board and carefully slide it out. Take care not to damage the SIMMs, especially the larger DIP SIMMs that some boards carry.[9]
  2. Find CR1 on the component side, just behind the DB-25 connector J3.[6] In the lead photo it is below the left-hand end of J3, beside R3 and C47.
  3. Fit the diode with its band at the end marked by the bar of the diode symbol printed beside CR1 (the right-hand end in the lead photo). On Apple's schematic the anode goes to +5VF and the cathode to J3 pin 25, TPWR.[2]
  4. Solder both leads and trim them.[6][7]
  5. Slide the logic board back into its rails, reconnect the logic board cable and the floppy drive cable, and refit the cover.[9][10]
  6. Plug the BlueSCSI into the SCSI port with no USB lead connected.[7]

Testing

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  • The PWR LED on the BlueSCSI lights when the board has power.[4]
  • Switch the Mac on. It should start from the image; the ACT LED blinks when the Mac reads or writes the emulated disk.[4][7]
  • Every time the BlueSCSI powers up it writes log.txt to the root of the card, recording the voltage the Pico board sees and the disk images it found. Read it on a computer if anything is wrong; it is overwritten at the next power-up.[4]
  • A flashing question mark means the image has no usable system or lacks a SCSI driver; try a ready-made image.[4]

Revision notes

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  • SCSI upgrade boards for the Macintosh 128K and Macintosh 512K, such as the Dove MacSnap and the Levco MonsterMac, use Macintosh Plus ROMs and work with the same Mac Plus configuration file.[1]
  • BlueSCSI's documentation describes a separate modification, the Clipper Plus, that adds an internal 50-pin SCSI connector to the Plus.[1]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Compatibility, BlueSCSI Documentation—link(accessed 2026-10-25) Mac Plus: "Requires some special configuration. Go to https://ini.bluescsi.com and click Mac Plus"; "The Plus does not have an internal 50 pin SCSI bus. DB25 only"; the Clipper Plus modification adds an internal 50-pin connector; "The Mac Plus does not provide term power. You will have to provide power via USB or do the diode mod"; Dove MacSnap and Levco MonsterMac use Plus ROMs and work with the Mac Plus .ini; "Debug is currently not supported when using System=MacPlus".
  2. ↑ 2.0 2.1 2.2 2.3 Apple Computer, drawing 050-0159-A, Schematic, Mac+ Main Logic PCB, March 1986: U6A 5380 SCSI, J3 DB-25 SCSI, CR1 1N4001 between +5VF and J3 pin 25 TPWR; reference designation table lists CR1 under "Not used". Hosted as File:MacintoshPlus LogicBoard Shematic.jpg.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Usage, BlueSCSI Documentation—link(accessed 2026-10-25) exFAT (preferred) or FAT32 card; naming convention HDxy_512.hda, ID defaults to 1, text after the block size ignored, 64-character limit; raw mode when no images are present.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 Troubleshooting, BlueSCSI Documentation—link(accessed 2026-10-25) Power the Pico from a USB adapter of at least 1 A; the Mac Plus does not provide termination power, so use a 1 A USB adapter or the diode modification; Class 10 or better cards; SD Memory Card Formatter "Overwrite format"; Windows hidden extensions; Pico LED 5 flashes = no SD card, 3 flashes = no valid images; PWR and ACT LEDs; log.txt written at every power-up with the voltage the Pico sees; flashing question mark and emulator images missing a SCSI driver.
  5. ↑ 5.0 5.1 Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition, Addison-Wesley, 1990, chapter 11, p. 378: "Because of power supply limitations, the Macintosh Plus and the Macintosh Portable do not provide power for termination resistors at the SCSI connector"; Table 11-1, pin 25 TPWR: "On the Macintosh Plus, this pin is not connected. On other Macintosh models, this line is provided for powering termination resistors only, not for powering SCSI devices." Hosted as File:Apple Guide to the Macintosh Family Hardware 2nd Edition 1990.pdf.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Mac Plus SCSI term power mod– rdmark, TinkerDifferent, 2 April 2023—link(accessed 2026-10-25) The Plus "has the requisite circuitry" if a diode is added at CR1 "right behind the SCSI DB25 connector"; 1N4001 50 V 1 A or another 1 A diode with similar voltage rating.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Restoring a Macintosh Plus and Convincing It to Work With BlueSCSI– Dan Schnur, Low End Mac, 8 August 2026—link(accessed 2026-10-25) BlueSCSI showed no lights; 1N5819 Schottky soldered at CR1 for its low forward drop, after which the BlueSCSI worked without USB power; [SCSI] System=MacPlus; image HD0_512-1GB-MacPlus-6+7.hda from the BlueSCSI image library booted.
  8. ↑ 8.0 8.1 Apple Computer, Macintosh Plus Service Source, hosted as File:Macintosh Plus.pdf (see Macintosh Plus Service Source). Troubleshooting, Symptom Charts/Miscellaneous (Continued) p. 11 (PDF p. 25).
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Apple Computer, Macintosh Plus Service Source, Take Apart, Logic Board pp. 7-9 (PDF pp. 33-35).
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Apple Computer, Macintosh Plus Service Source, Take Apart, Cover pp. 1-2 (PDF pp. 27-28).
  11. ↑ Hardware, BlueSCSI Documentation—link(accessed 2026-10-25) DB25 2024.01a: "PCB is now only as wide as the DB25 connector"; DB25 2023.06a: "termination is now always on".
  12. ↑ bluescsi.ini, BlueSCSI Documentation—link(accessed 2026-10-25) System, section [SCSI]: presets Generic, MacPlus, MegaSTE, X68000; "MacPlus - Mac Plus: Apple Macintosh Plus and similar"; a preset "applies quirks, timing, and other settings automatically".
  13. ↑ bluescsi.ini Generator, BlueSCSI—link(accessed 2026-10-25) The Mac Plus quick preset sets System = MacPlus in the [SCSI] section.
  14. ↑ 14.0 14.1 Macintosh Plus - Trouble after doing SCSI Term Diode Mod, TinkerDifferent, June-July 2024—link(accessed 2026-10-25) One owner fitted a 1N4007; after several good boots the speaker became quiet and the machine stopped starting; other members reported the modification working on their machines and suggested logic board or connector faults; no cause was found in the thread.


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