The CIA World Factbook 1992/k3bimage.iso/sel/04/0037/fdcable.doc

Text File | 1991-12-02 | 10.4 KB | 187 lines

The message below was originally posted as an explanation of why it was
necessary to set both floppy disk drives up with the same drive select
jumpers when you want one to be drive A: and the other to be drive B:.
It explains in detail the whys and wherefores of twisted floppy cables and
how to set up the drive selects on the floppy drives.  The version below has
been edited slightly to make it more generally applicable.

Written and uploaded by Tom Rawson (73577,243).

Michael ... Glad you got your problem fixed.  Here's the way it works to make
you set both drives for DS1:

(1) The controller puts out two signals on the drive cable, called "Drive
Select A" and "Drive Select B", to select the appropriate drive when issuing
commands. In other words, if (for example) the controller sends a "step"
signal to the drive to move the heads in or out one track, it uses a single
wire in the cable - called, strangely enough, "Step" - to send the command. A
pulse on this wire means "move the head in or out" (which direction is
determined by another line, "Direction"). The Drive Select A & B signals
determine which of the two drives will respond to the command, A: or B:. That
way the controller doesn't need a whole bunch of separate command wires for
each drive - just one set of command wires, and different drive select wires
to say which drive should respond to the commands.

(2) The "DS" jumper on the drive determines which drive select wire the drive
will pay attention to:  if you set it to DS1 it looks at Drive Select A (and
thus - lo and behold - becomes drive A:); if you set it to DS2 it looks at
Drive Select B, and is then drive B:. (Some drives number the drive selects 0
and 1 instead of 1 and 2, but the same principle applies). Given all this, if
things were organized in the apparently logical way, with all the wires hooked
up as described above, your advisor who wanted you to set one drive as DS1
and one as DS2 would have been correct; the drive set up as DS1 would have
drive A: and the one set up as DS2 would have been drive B:.  But we forgot
something:  the cable.  Here's where the fun starts.

(3) Take a look at your floppy cable - the ribbon cable from the controller
board to one drive then the other.  If all the wires were straight things
would behave as described above.  But they aren't:  the cable either starts at
the controller, and at some point several wires (7, to be exact) are split out
of the cable, twisted 1/2 turn, go through the middle connector, twist back to
their normal alignment, and go on to the end connector; OR, the wires are
split out of the cable AFTER the middle connector and go into the end
connector twisted.  In either case the effect is the same:  one connector has
those 7 wires in their normal alignment, and the other has them twisted 1/2
turn.  (Most cables are of the former type, with the twisted section going
through the middle connector).

(4) OK, what is the effect of this funny Moebius twist in your cable?  Well,
it just so happens that those 7 wires are pins 10 through 16 of the connector
(you can count yourself - the marked wire, usually red or blue, is pin 1).
The effect of flipping them over like that in one connector is to put the
signal that was on pin 10 in the cable on pin 16 of the connector, and vice
versa; the same swap is done with 12 and 14.  This means that whatever signal
the controller sends on pin 10 comes out on pin 16 in that one connector, and
vice versa; again, the same is true of pins 12 and 14.  It further turns out
that the Drive Select A and B signals are pins 12 and 14 of the cable (I'll
get to pins 10 and 16 below, and the odd-numbered pins are all grounded, so
swapping them around doesn't matter).  SO, when the controller sends out a
Drive Select A signal it comes out on pin 12 of the cable - or pin 14 of the
connector with the cable flipped.  When it sends out a Drive Select B signal
it's vice versa.

(5) All the drive select jumpers on the drive do is pick which pin (12 or 14)
the drive looks at to determine whether to respond to commands from the
controller.  If you jumper both drives as DS1 they will both respond to
signals on pin 12.  BUT, due to the twist in the cable, the drive on the
connector with the twisted wires gets a signal on pin 12 of the drive that
originally was on pin 14 of the cable; that's drive select B, and it will
respond to that one.  The drive on the connector with "normal" wire alignment
will also respond to the signal on pin 12, but that comes from pin 12 of the
cable, and that's Drive Select A. Voila! You have both drives jumpered as DS1;
one responds to Drive Select A; the other responds to Drive Select B - and
that was the object all along.  The twist in the cable does the trick.
Furthermore, since it is usually the _middle_ connector where the twisted
wires are connected, and the twisted wires are for drive B:, usually the drive
on the middle connector is B:.

(6) Note some other effects of this scheme:  If you jumper the drive on the
connector with twisted wires as DS2 and the one on the connector with normal
wiring as DS1 - as you probably did at first - they will both respond to Drive
Select A.  If you jumper both drives as DS2 it will also work, but it will
swap things so that the connector where the wires are twisted will be drive
A:, and the one with normal wiring will be drive B:.

(7) Pins 10 and 16 of the cable are Motor Enable A and Motor Enable B
respectively; these lines start the drive motor _and_ turn on the red access
light.  They are swapped due to the cable twist EXACTLY like the Drive
Selects, and everything said about the Drive Select lines above applies to the
Motor Enable lines as well.

The possible arrangements of drive selects and connectors can be summarized
as follows; the letters (A, B) in the table indicate which drive letter the
drive will respond to when the system is running:

For systems where the floppy drive cable has a twist in some of the
connectors:

        Connector:      With twisted wires      Wires aligned normally
                        (usually in the mid-    (usually at the end
                        dle of the cable)       of the cable)
        Drive Select
        ------------
            1                   B                       A

            2                   A                       B

The most common arrangement is that shown in line 1 of the table:  BOTH
drives jumpered for DS1; the drive on the middle connector is B: and the one
on the end connector is A:.

Note that, as mentioned above, some drives have drive select jumpers marked
"DS0" and "DS1" instead of "DS1" and "DS2"; in that case everything above
applies, but you have to use "DS0" where I said "DS1" and "DS1" where I
said "DS2".  This variation is due to individual manufacturers' labeling
preferences as there is no particular standard.

For systems where the floppy drive cable DOES NOT have a twist in some of the
connectors (this is rare but I have included it for completeness):

        Connector:              Either

        Drive Select
        ------------
            1                      A

            2                      B


Notes on "terminating resistors":  A terminating resistor is a small chip,
usually with 14 or 16 pins and often a different color than the usual black
of most ICs (grey, blue, yellow, etc.).  Its purpose is to stop signals in
the cable from "ringing" due to reflections; I suppose you can view it as
like putting some soft material in the cap at the end of the pipe so it
doesn't bang around so much when the water goes on and off.  There should
be one terminating resistor chip on the board at each end of the floppy
disk drive cable.  The controller card always has one of these chips, so
you don't have to worry about that end.

Most disk drives have a socket for this chip.  Some come with the chip
installed, some don't.  The main thing you have to know is that the termina-
ting resistor ALWAYS goes on the LAST drive on the cable.  If you only have
one floppy drive, it goes there.  If you have two, there should be a
terminating resistor installed in the LAST drive ONLY.  Note that the only
thing that matters here is where the drives are physically located on the
cable; which drive gets the terminating resistor has NOTHING to do with
which one is A: or B:, since the latter can be modified through changing
the drive select jumpers without physically moving the drives on the cable.

If you have are installing a second floppy or replacing a floppy drive, you
want to be sure that you have one and only one terminating resistor,
installed in the last drive in the cable.  You don't want one in each drive,
nor do you want one in the middle drive and none in the last drive (though
I have seen the latter configuration work, you can't depend on it).  To
accomplish this you need to figure out where the resistor is on your
drive(s), and whether it's installed.  The position varies widely on
different types of drives, so you have to depend on the drive documentation,
the dealer, the manufacturer, or others who know, but it is usually somewhere
vaguely accessible and visible - like inside a hole cut for that purpose,
or near the back edge of the circuit board on the drive (though it probably
won't be as accessible or visible as you'd wish).  On some drives it is near
the drive select jumpers.  I have heard that some drives don't actually
allow you to remove the resistor, but instead have a jumper that activates
or deactivates it; I believe this arrangement is quite rare.

Note that if you have one drive that you are pretty sure has a resistor and
one that doesn't, you can simply install the one with the resistor at the
end of the cable and the other one in the middle, and don't bother with
moving the resistor around.  Then set the drive select jumpers according
to the table above to get the drives appropriately arranged as A: and B:.

For the technically curious, the terminating "resistor" is actually a
"resistor pack" which contains several individual resistors.  The circuit
is such that the signals in the cable which are "terminated" by this
resistor pack are "pulled up" to a positive voltage and/or "pulled down"
to ground through resistors.  I believe in most cases the resistor packs
used for the floppies are the combined pull up/pull down type, but I'm
not positive.