step signal on 8" floppy drive

From: Lawrence LeMay <lemay_at_cs.umn.edu>
Date: Wed Apr 14 22:05:34 1999

> >
> > Hmm, perhaps wiring an external Terak floppy drive to a PC is a bit
> > more complicated than i thought.
> >
> > I forgot that the Terak main unit controls the power switch on the
> > external drive somehow. Looking inside teh case, it looks like the
> > signal wires that correspond to "Step" have a special connection
> > in addition to going to the floppy drive. This must be what is
> > controlling the on/off switch, but i'm curious now as to how the
> > "Step" signal is supposed to work. Does anyone see how this might
> > be used to control the power?
>
> IS it _only_ the step wires that are used like that?
>
> What do they connect to? How complex is the (presumed) PCB? COuld you
> trace out the schematic?
>
> My guess is that as the Step wires are normally high and pulse low
> briefly to move the head, the power switch circuit detects if Step is
> stuck low, and assumes the computer is turned off if that's the case.
> Sort of thing you could detect with a TTL monostable chip.
>
> >
> > And do you think i'm completely nuts to connect this thing to my
> > PC, ie, is it likely to burn out the motherboard ;) ... Ah well,
>
> Should be OK. It's all TTL level signals (and open-collector ones at
> that), so you shouldn't do any damage..
>
> Of course modern FDCs in PCs have the cable drivers built into a large
> ASIC (along with the rest of the FDC, IDE port, serial ports, etc), which
> makes repairing a blown driver a little hard.
>
> -tony
>

Hmm, The circuit is reasonably complex, in that it keeps connecting to
various power circuit connectors. But this is what the start of the
circuit appears to do.

The 2 Step wires appear to connect to the wire coil that controls
a DPDT relay switch. The coil part of the diagram on the relay
says 12 VDC, though apparently we're feeding this with 5 VDC?

Even though the switch appears to be DPDT, its being used as a DPST.
When the relay is activated, two separate connections are made, and
these go to the power and other connectors, amoung other things i'm
sure. Is it reasonable to think that the Step current is enough to hold
the relay activated, even when its briefly interrupted? Seems wrong
to me. OK, on further examination one switch appears to connect one of
the AC wires to the rest of the board, and the other might act in a
similar fashion. Case ground (3rd pin from the power cable) is always
connected as expected. So, this relay does as we expect, it turns
the unit on and off.

So, is there some sort of a digital latch circuit? Well, I dont see
much in the way of digital stuff. There are 2 small transistors and 2
IC's reasonably near this relay, with 2 more IC's a bit farther away, and
that appears to be it besides a LOT of resistors, capacitors, diodes, and
other power circuit stuff.

First 2 IC's are LM139J and LM124J. Other two are both SG3524N (regulating
pulse width modulator, whatever that is).

Anything ringing a bell?

-Lawrence LeMay
Received on Wed Apr 14 1999 - 22:05:34 BST

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