From: zappe@mikro.ee.tu-berlin.de (Harald Zappe)
Subject: Re: Low Voltage on Pin 38
Organization: ZRZ/TU-Berlin
Date: Mon, 14 Sep 1992 22:55:28 GMT

In <1992Sep14.142722.22352@mccc.edu> pjh@mccc.edu (Pete Holsberg) writes:

>In article <zappe.716415197@tubue> zappe@mikro.ee.tu-berlin.de (Harald Zappe) writes:
>=In <1992Sep13.000854.11526@mccc.edu> pjh@mccc.edu (P. J. Holsberg) writes:
>=
>=>I'm trying to connect up a device that's powered from a SCSI device --
>=>WangDAT 1300 to be precise -- and it gets its power from pin 38,
>=>termpwr. Unfortunately, the device needs 5V and the 1300 is putting out
>=>only about 2V.
>=
>=To avoid electrical reflections, the SCSI-cable has to be "term"inated
>=with resistors, which are pulled up to a positive voltage at pin 38.
>=The only purpose of this pin 38 is just to 'drive' the resistors
>=(TERMimator PoWeR). So it doesn't matter if the voltage sinks down.
>=If you don't fear, try to drive a LED too. But more, I think is
>=impossible.

>Wouldn't it be more likely that each data pin in the cable has a
>resistor between it and pin 38, and that pin 38 would have +5V on it
>from the power supply? I think that's the usual way lines are terminated.

I don't see an inconsistency. The "quality" of this 5V power supply can
be relatively bad. It may break down when it is connected to 18 data pins.
I don't know what's installed behind the fuse on my Adaptec 1542b.

>=PS: Pay attention that there are only resistors at the physical end of
>=the SCSI-cable. Most SCSI-drives come with resistors installed. If you
>=connect more than 1 device, you'll have to remove all of the resistors
>=EXCEPT those of the LAST drive.

>This device, a WangDAT tape drive, has a second SCSI connector and it
>has the terminator plugged in. (I should check the continuity of that
>connector.) The thing I'm hooking up is a PC-parallel-port-to-SCSI
>adapter, that will drive a SCSI device through the parallel port of a PC.

Hmmm, ... parallel port ? ... From where does the converter get its
power supply ? I had a similar problem with the missing 5V pin at the
parallel port. The solution is still not finished. There were three
possibilities: The keyboard connector (pin 5), the joystick port (pins
1 and 9), or an extra wire comming directly from the main power supply
(which I used for semi-external drives. Meanwhile the drives are mounted
inside the case, but I still have the connector with +5V,Gnd and +12V at
the back panel).

Or does the adapter come with its own line transformer? Then I would
assume that this is a nice little peace combined with the 2 AC-plugs
in 1 case (the transformer, not the adapter), with a maximum current of
(let's estimate) 500..800 mA.

My Syquest changable media drive nees approx. 1.2A at the 12V line to
spin up. I can imagine that a tape drive has similar requirements.
Where is the origin of the power? Where are the 12V? Sorry, I can't see
the beast. (It's late, lets continue later.)

good night, Harald

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