Tech Flashback: SCSI Controllers

Look at that port. It looks just like a regular DB-25 female connector. If it didn’t have that yellow label beside it, one could easily mistake it for a PC Parallel Printer port. But it’s not. It’s a SCSI port.

If there’s anything this blog has shown, it’s that technology evolves at an unrelenting pace. New things keep coming, and old things become obsolete and forgotten. SCSI, standing for Small Computer Systems Interface, is one of those interfaces which is all but irrelevant. SCSI, pronounced scuzzy, is anything but sexy (apparently the designer intended it to be pronounced sexy).

SCSI was a confusing bus to get used to. It went through many iterations, with different speeds, cable-length restrictions and device number restrictions. It was a “high performance” bus (for the time), but the additional smarts made it quite expensive. It was virtually identical for the external and internal variants, bar a few connector differences. Internally, 50 pin IDC connectors, similar to IDE but longer were normally used for the single ended SCSI variants, with different 68-pin connectors used for differential SCSI. Externally, it’s not uncommon to see the DB-25 connector, Centronics/Honda connectors, etc. The bus itself was single-ended (later, differential), and required the installation of terminators at each end, and the setting of device ID (0-7). Everything was daisy chained, and most adapters had the internal and external buses connected together. Given all these things, it was a rather difficult bus to get the hang of, at least initially, so that installing your scanner didn’t break your hard disk etc. Device to device transfers without host intervention were possible, however, it was mostly used for device to host/host to device transfer. CD-ROM drives, CD-Writers, Hard Drives, Scanners and ZIP/Jaz and Hard Carts were just a selection of devices that SCSI was used for.

Imagine my amazement when I managed to pull out a few SCSI host controllers out of a box I had long forgotten about!

This is a Future Domain TMC-845 9136AV based ISA 8-bit SCSI Controller. It’s dated back to 1989 – making it just as old as I am. There is the 50 pin header for internal SCSI devices and the 25-pin DB-25 connector for external devices. A few jumpers set the IRQ and I/O addresses. The empty IC-socket row just below the 50-pin internal header is for termination resistor packs – if you wished to terminate the bus and use the card as internal-only or external-only, you would fit the resistor packs to those pins and be done with. Later cards/devices had auto-termination and other fancy things like that.

This is another Future Domain based card, using the 9214AV chip. This one is produced by a third party vendor, but was almost certainly sold either bundled with or for use with an external SCSI CD-ROM drive. The giveaway is the RCA connectors on the back panel, which would let you hook up the RCA connectors on your external case to them so that the audio would be routed through the sound card. An inductor for those RCA connection can be seen too, to suppress RF noise – good design! All the above are likely to be SCSI-1 cards, with a theoretical 5MB/s transfer rate, but due to the use of 8-bit ISA, it’s not likely that users saw any more than about 1MB/s of real throughput.

This is a Domex DMX3191D, with a more modern PCI interface. Notice how this card has no internal interface? The resistor packs come pre-soldered right next to the external DB-25 connector. My guess is that this card was bundled with an external drive or scanner.

Finally, one of the kings – Adaptec. This company was pretty much synonymous with SCSI controllers. This is an AVA-2902I – an Internal-only card, missing its backplate. Also notice the termination resistors pre-soldered in. This one might have been bundled with a CD-burner, being an SCSI-2 card with a theoretical 10MB/s transfer rate.

Interestingly, when I began learning about computers, the wisdom was that parallel interfaces are faster than serial interfaces. Parallel interfaces would transfer many bits at the same time, whereas a serial interface could only transfer one. Oh how that wisdom has changed – there are many electrical challenges (signal skew/delay, interference/crosstalk, reflections) that come about when trying to run high-speed parallel buses that serial has now become faster and more used. Think of SATA vs PATA, PCI-E vs PCI, USB vs Parallel Port. We’re moving more towards high speed serial, and or sometimes, multiple high-speed serial lanes side-by-side (which isn’t exactly the same as parallel if the lanes operate independently, but do share some of the difficulties).

Also interesting is the name – SCSI standing for Small Computer … SCSI itself saw its main use in enterprise, high-throughput systems. Its expense was no problem for a place where performance and reliability was the main driving force. Despite this – SCSI signalling itself is now pretty much defunct, it leaves us a legacy in the command set but that’s about it. Enterprise hard drives are SAS (Serially attached SCSI) which uses a SATA style physical layer, transporting SCSI based protocol.

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