Mac IIfx and cd-roms

When those Macs were popular, folks who wanted a CD drive used the Apple CD600e, an external enclosure with a SCSI interface. Plug it in with a "System" SCSI cable (25 to Centronics-50), put the BLACK IIfx Terminator on it, and you're good to go. There was also a (slower) 300e.

If you can't find one, email me. I think I still have two.

1) There is no bay for it in a IIfx. Are you going to run the computer with the lid off?

2) The CD drive's built-in termination does not contain the extra de-glitch circuit found in the black terminator, which I believe only appeared in Centronics-50 pass-through form factor.

From FWB's Guide to Storage ©1996:

"The Macintosh IIfx is in a class by itself. One of its features is a SCSI chip that provides faster data transfer rates than possible with earlier Macs. This necessitates the use of a combination of the following three termination parts.

• Apple SCSI Cable terminator II. This is a black external terminator that ships with the Mac IIfx. It includes a "glitch eating" capacitor, as does the filter. (See explanation below.) Only one is used on the SCSI Bus.

AND either

• Internal SCSI Termination Block. This provides the internal termination for IIfx machines without internal Hard drives, and is installed by Apple when shipped.

OR

• Internal SCSI Filter. This provides filtration of SCSI lines for Macintosh IIfx Internal drives os systems that shipped prior to March 19, 1990. It was installed by Apple when shipped.

Macintosh IIfx termination configuration is simple. There is one terminator installed internally in the Macintosh itself, and another externally at the end of the SCSI chain.

The reason these parts are required is the IIfx's SCSI chip thinks that glitches on the Request line are genuine signals. The internal SCSI filter is actually a capacitor that may be though of as a glitch-eater. The glitches occur when a majority of data lines change their state simultaneously, which drains the terminator power line (TPWR line) and consequently causes a power spike on the Request line. The solution is to have the internal SCSI filter provide the TPWR line a little extra current when needed."

In the day when those were popular, the major knock-off for third-party drives was that they were not Mac-bootable. Not every SCSI CD drive is Mac-bootable.

If you make a drive bay by eliminating the extra height above the SIMM sockets, you cannot use the high capacity SIMMs that make living with the Mac IIfx pleasurable. (I think I have a spare set of those, too, if you want them.)

The single SCSI bus in the Mac IIfx can support both Internal and External segments. Each additional device extends the metallic pathway that is the SCSI Bus -- the signals are not normally re-driven. Properly-designed External enclosures have two connectors to allow the Bus to be extended again, or to hold the terminator.

The choice of the DB-25 connector for the External connector at the CPU end is unfortunate. It forces the use of a Mac "System" cable -- a DB-25 at one end, and typically a Centronics-50 for the first external device. Having a different cable there, rather than the same type of cables throughout, reduces your flexibility.

Use of cables (like RS-232 cables) not intended for SCSI devices punishes the performance of the entire Bus and every device on it. Cables under a few feet in length are considered short -- this Bus can support a total cable length (including the extra about 1 ft. inside each external enclosure) about six meters (nearly 19 feet) when the external terminator is used. Generally speaking, shorter is better.

Appropriate termination of the last device on the internal chain, and the last device on the external chain is essential for proper operation of the Bus. In the case of the Mac IIfx, the external chain requires the BLACK terminator. Without proper termination, the negative-true Bus signals "float" rather than rather than being "pulled up" to the inactive state, and are also subject to ringing and noise. An improperly terminated Bus will cause the Mac to Hang.

One must Never plug and unplug SCSI devices with the power on. Serious Damage can result.

The Mac IIfx SCSI Bus supports up to 7 SCSI devices total, using IDs 0 to 6. Device 7 is the SCSI controller on the motherboard. The standard configuration uses device=0 for the Hard Drive. It is customary to assign a built-in CD drive device=3.

The remainder of the IDs are available. Unlike the PC world, devices intended for the Mac market are generally very well-behaved, and it is entirely possible to use up all the ID numbers and have a SCSI Bus that functions well. That typically meant a small stack of external device enclosures next to your Mac, with External cabling running from box to box.

This article may help with the vocabulary and help present "The Macintosh point of view":

9387 -- Connecting SCSI devices to your Mac