External CD-ROM support interfaces: Which host controller technology historically supported connecting an external CD-ROM drive to a PC?

Introduction / Context:
Before USB became ubiquitous, external storage and optical devices often relied on interface standards that supported external cabling and device addressing. Identifying which bus allowed reliable external CD-ROM connections is essential for maintaining legacy systems and understanding hardware evolution.

Given Data / Assumptions:

• We are comparing historic disk interfaces: SCSI, ESDI, MFM, and vendor-specific schemes like ARLL.
• The focus is on external (out-of-chassis) device support with standardized cables and termination.
• CD-ROMs were common peripherals during the SCSI era.

Concept / Approach:

SCSI (Small Computer System Interface) supported multiple devices on a shared bus, device IDs, termination, and standardized external connectors. ESDI and MFM were primarily internal drive interfaces without mainstream external device ecosystems. Therefore, external CD-ROMs commonly used SCSI, particularly in workstations and high-end PCs.

Step-by-Step Solution:

Verification / Alternative check:

Period documentation confirms SCSI as the de facto external peripheral bus pre-USB, with terminators, IDs 0–7 (narrow) or 0–15 (wide), and external Centronics/DB-style connectors.

Why Other Options Are Wrong:

• ESDI/MFM: mainly internal HDD technologies; no standard external device ecosystem.
• ARLL: not a mainstream standardized external interface.
• None of the above: incorrect because SCSI explicitly supports external devices.

Common Pitfalls:

Confusing parallel-port external CD-ROMs (later options) with true bus-level external support; those were slower and not typical for performance use compared to SCSI solutions.

Final Answer:

SCSI