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Why the RX50 floppy arrows?

Chuck(G)

Chuck(G)

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Many floppy disks made specifically for RX50 use (e.g. Verbatim MD-577-01) have a label stuck onto the bottom edge of the floppy with a red arrow pointing out. More curiously, both sides of the floppy have the arrow.

Was this labeling feature DEC-specified? Exactly what purpose does it serve?
 
Since the bottom disk on an RX50 goes in upside down, the arrow probably matches the arrow on the RX50, which are on different sides of the drive, top and bottom.

It's been 10 years or so since I sold my Rainbow 100+ systems, but that's what I remember.
 
That's what I thought, but wasn't sure. I imagine that most people just bought generic "360K" floppies and used them after someone figured out how to format them.
 
The arrow on the media aligns with the stripe on the RX50 drive. It's that simple.

But it's worth a few more words on the RX50 drive. There are two slots for disks, but only one spindle and one carriage that moves the heads. RX50K media is single sided, and the write surfaces face into the middle of the drive, where the carriage is that moves the heads. The two heads (one for each disk) move together.

So the arrow aligning with the stripe ensures the correct surface is facing the head.

Lou

PS. the DECMate II and III that I will bring to VCF East will have RX50 drives if anyone wants to see.
 
Okay, just that simple. Reminds me of the old Altos 8" systems. The drives are mounted upside-down--so if you find a SS floppy with the label on the "worng" side, you know where it probably came from.
 
Since there's a single spindle and one floppy is flipped over, does the controller need to read/write one floppy backwards compared to the other? Or, does the drive reverse the spindle rotation when accessing the other disk?
 
NF6X said:
Since there's a single spindle and one floppy is flipped over, does the controller need to read/write one floppy backwards compared to the other? Or, does the drive reverse the spindle rotation when accessing the other disk?
Click to expand...

The lower drive element is effectively upside down. By inverting the diskette in the lower drive element, the common motor moves both diskettes in the correct manner and disks written by one element can be read by the other.
 
krebizfan said:
The lower drive element is effectively upside down. By inverting the diskette in the lower drive element, the common motor moves both diskettes in the correct manner and disks written by one element can be read by the other.
Click to expand...

If the spindle is a solid shaft with no gearing to make the two hubs rotate in opposite directions (an assumption, as I haven't seen the innards of an RX50 yet), then flipping the lower disk over makes its recorded surface move in the opposite direction under its head vs. the upper disk. I just experimented with some props to make sure I was thinkin' right. ;)

So, either the electronics would need to reverse the data for one diskette, or the drive would need to change the spindle motor direction. If the two diskettes were inserted in the same direction and the head carriage did a reach-around to get to the recorded surface of the lower disk, then no reversal of the spindle or data would be needed; the two diskettes would just get clamped to the spindle from opposite sides.
 
Bitsavers has a diagram of the RX-50 which shows how the single spindle handles both disks and read write heads are set in the middle. http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/disc/rx50/

But yes, the disk is rotating backwards but the lower read-write head was inverted to correct for that problem. The upper drive element writes to the lower surface of the disk like normal single sided drives. The lower drive read write head is mounted above its disk and writes to the same surface when the disk is inserted upside down. It is a very strange system.
 
Inverting the read/write head to compensate for the disk surface moving in the opposite direction doesn't change the fact that the flux transitions will arrive at the head in reversed order on one of the disks. With one disk flipped and both clamped to a common spindle, one head will need to read/write each sector from first bit to last, and the other will need to do so from last bit to first. It seems like a screwy approach to me. It should also complicate locating sectors if they neither used hard sectored floppies nor track-at-once writing, since the more common approach of searching for a sector header with the right number wouldn't work on one disk. Though, I suppose the controller could search backwards for sector header N+1 to find sector N on the reverse-spinning disk.
 
Oh I'm such a moron. I threw away several of these 20 years ago thinking they were worthless :(

It seems to me that the simplest thing to do would be to reverse the spindle motor rotation.
 
KC9UDX said:
It seems to me that the simplest thing to do would be to reverse the spindle motor rotation.
Click to expand...

From EK-PC350-TM-001_Professional_300_Series_Technical_Manual_Dec82.pdf, which devotes an entire chapter to the intricacies of the RX50

-----
10.1.3 Physical Description
Figure 10-2 shows the RX50 drive. The front bezel has two access slots with swinging doors to insert or
remove diskettes. A head load indicator for each diskette slot lights when that unit is busy.

Internally, the drive has two counter-rotating spindles. The spindles are belt-driven by a single dc motor /
tachometer combination.
.....
10.3.1.2 Spindle Drive Mechanism - The drive contains two counter-rotating spindles, one for each drive
side. A single 12 V dc spindle motor-tachometer combination drives the spindles with a belt. The motor
operates at 1800 rpm while the spindles rotate at 300 rpm.

The motor control module keeps the motor rotating at a constant 1800 rpm. The module compares the
tachometer pulses coming from the motor to a constant reference. This comparison determines the motor's
current requirements provided by the motor control module. For further information on the motor control
modules see Section 10.3.3.
-----

Yes, two counter-rotating spindles. QED :->.
 
The way it works can be seen in the second drawing. The belt wraps around both spindles. So, I was wrong: that is much simpler than reversing the motor.

Actually, it's kind of ingenious. It's certainly not the kind of engineering anyone does today!
 
Counter-rotating spindles makes a lot more sense to me than dealing with a reversed data stream. So, it's basically a double sided drive where each side is on a different floppy. :p
 
jackrubin said:
Xemer? Whatever they did sucked, so I wouldn't be surprised.

Jack
(feeling particularly grumpy today - we need Fred to check in here every now and then)
Click to expand...

Frend still hangs out at cctalk. Herb Johnson might also remember. My own wetware has decayed enough that I can't be sure.
 
I cant tell, but that may be a different arrangement. Purely a guess, but that may be gear driven instead of belt driven.
 
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