kaufman@polya.Stanford.EDU (Marc T. Kaufman) (04/04/89)
In article <839@orbit.UUCP> pj@pnet51.cts.com (Paul Jacoby) writes: > ...I have an external >Mirror 40 meg (SCSI 4) and 40 meg tape drive (SCSI 1). I am finding that when >I have the external drive/tape plugged in but NOT powered on, I cannot boot >the SE/30. At all. Just a grey screen--not even a flashin disk. This sounds >reminiscent of the older Pluses...or do I just have a termination problem? It is >JUST< a termination problem. On Macs prior to the MAC II, no power for terminators was brought out to the SCSI port, so the device was responsible for powering the termination. If the terminators are unpowered, they pull down the lines and prevent SCSI operation. On the MAC II (and, I presume the SE/30), +5 volts is provided on pin 25 of the SCSI connector to power terminations. Peripheral device manufacturers "knew" that the MAC (+ and SE) did not power the terminators, so they power them from the internal device supply. I have discovered that, quite often, they did not provide a diode to allow powering the terminations from the BUS. Also, they do not provide (through a diode) power TO the bus -- which means that if you remove the internal termination resistors so you can daisy-chain devices, and put an external termination on the end of the chain, the system will not work unless you are on a later MAC that provides SCSI terminator power. (sigh!). They saved about $0.60. Marc Kaufman (kaufman@polya.stanford.edu)