At 10:32 -0800 11/20/11, anthonyschr wrote: >Ah. I had understood that the Plus simply did not feature built-in >termination, requiring the user to use a terminator. Recently, I've >been using a black inline terminator with the apple logo on it, and >have not been "daisy-chaining" the SCSI devices. I have several times >successfully mounted the PowerUser drive on the Plus using this >arrangement, but I haven't been able to do so with the PocketHammer. > >Do I need a terminator that connects to the unused SCSI port on the >device? Does it need to be powered? >
Both ends of any SCSI string need to be terminated. Each end of a single data conductor gets two resistors, one going to plus 5 volts and the other to ground The idea is to prevent reflections that send signals back into the cable unless the terminating resistors match what is called the characteristic impedance of the wires, about 110 ohms for most plain pairs. 220 ohms to +5 and another 220 ohms to ground does that but the resistors were not identical. The source of the +5 volts is called terminator power. The early Macs, including the plus, do not provide it. Many SCSI devices do though it's often necessary to install a jumper to make sure it gets on the proper wire in the SCSI cable. The classic Macs use a 25 pin connector which was never a SCSI standard. That requires 50 pins with a separate ground wire for each data line. Apple cheats a bit by sharing ground pins. That makes it pretty tricky to create a terminator that works with the 25 pin connector. One can assume that Macs and other devices that have only one 25 pin connector (Zip drives) provide the 110 ohm terminator. That does not imply that they provide the 5 volt terminator power though. When a device has two 50 pin SCSI connectors it may, or may not provide terminator resistors or terminator power. At one time it was absolutely necessary to plug either another SCSI device or a terminator into the second 50 pin connector. More modern devices try to make it simpler by providing switch or jumper selected termination. Some older disks had three 10-pin inline resistor arrays that could be plugged into the circuit board or not. The overall effect was the opposite of making anything simpler than plugging in a terminator. You now have to read the manual for the device and you probably can't find it. Apple also had special terminator plugs for the Mac IIFX. They ran a bit hotter and were identified by being black plastic instead of the standard gray. -- --> From the U S of A, the only socialist country that refuses to admit it. <-- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to vintage-macs@googlegroups.com To leave this group, send email to vintage-macs+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
