Sandra Warnken wrote: > You need to have term power at the end of the chain, not on each device. You are confusing "termination power" with "termination". A SCSI bus is supposed to be terminated at both ends. In the case of the Apple II, this means that the bus should be terminated at the controller and at the last drive. (For a computer which has internal SCSI devices, such as a Mac, the last internal device and last external device should be terminated. I think that the Mac also automatically terminates the bus at the motherboard if either the internal or external connector is unused.) The purpose of the termination is to prevent signal reflections at the ends of each signal wire. In effect, the terminators provide a "dampening" effect, swallowing signal pulses that reach the end of the wire. They also provide a clean "idle line" condition by holding the bus at a reasonably well defined voltage. With the simple "passive" type of terminator, this basically means that each signal wire has two pairs of resistors connected to it, one pair at each end of the wire. One resistor goes to ground and the other goes to the TERMPWR line, which is supposed to provide power for these resistors. An active terminator provides more reliable termination than a passive one, but it still needs the TERMPWR signal to get power. At least one device on the SCSI chain must provide a +5V signal to the TERMPWR line. It doesn't matter which device does this, and it doesn't matter if multiple devices do this (the one with the highest voltage output will "win" and get to supply termination power). It also doesn't matter where the device(s) supply termination power are physically located in the chain. Each device that supplied termination power is supposed to use a diode and fuse arrangement so that the TERMPWR signal doesn't try to supply power back into the device (and to protect against electrical faults). The original Apple SCSI card is not terminated on the card, which means that if you have multiple devices connected, you should have a pass-through terminator between the SCSI card and the first device, plus a normal terminator after the last device. (Alternatively, internal termination can be used on the first and last device.) If you only have one device connected, it should have internal termination, or external termination after the device. The Apple High-Speed and RamFast SCSI cards _are_ terminated, so you should only have one external terminator, connected after the last device on the chain (or use internal termination on that device). Neither Apple SCSI card originally provided power to the TERMPWR line. Late in the production run, Apple did a tack-on modification to the high-speed card, adding a diode, which allows the card to provide power to the TERMPWR line. The original card could be modified in the same way. The RamFast SCSI card can supply termination power (I think it has a DIP switch or jumper to select whether or not it does this). My personal feeling is that it is not a good idea for the Apple II to provide termination power, because it can draw quite a lot of current - almost the entire budget for the +5V rail going to the slots in the worst case. My preference is to ensure that at least one of the SCSI devices connected to the card is providing termination power. This avoids placing a heavy load on the Apple II power supply. If termination is not set up correctly, you can get all sorts of weird effects. For example, if nothing is providing termination power then the terminators acts as signal dampers, and are likely to cause communication errors over the bus. You may find that the bus works by removing the termination, though this can cause interference due to signal reflections. SCSI can get very weird with badly behaved devices, cables or connectors, and you may find that you can only get a particular combination of devices to work by arranging them in a specific order and/or breaking the termination rules (e.g. adding extra terminators in strange places, or disconnecting one or both terminators). -- David Empson dempson@actrix.gen.nz Snail mail: P O Box 27-103, Wellington, New Zealand