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wayne2 - The Percom Doubler - hay Shorty look at this
01-04-2014, 06:25 AM #1
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
I was talking to Shorty about some old electronic stuff and on a whim I googled an old project of mine. This is what I got. It was 1980.

The Percom Doubler

The Percom Doubler was the first successful double-density add-on for the TRS‑80 Model I. It was introduced in 1980 by Percom Data Company for an initial price of $219.95. The Doubler hardware was designed by Wayne Smith and Harold Mauch (president of Percom) and the accompanying software was written by Jim Stutsman.
Origins

The Radio Shack Expansion Interface, introduced in 1978, added floppy disk support to the TRS‑80 Model I. The floppy disk controller used in the Expansion Interface could read and write in single-density, also known as FM (frequency modulation). Using single-density, up to 87K could be stored on a 35-track single-sided disk or 100K on a 40-track single-sided disk.

One improved modulation scheme was double-density, also known as MFM (modified frequency modulation). Despite the name, double-density didn’t quite double the capacity over single-density. Using double-density, storage capacity increased to 180K on a 40-track single-sided disk.

Most people at the time thought it would be impossible to support double-density on the Model I without adding one of two things:

increased speed. At 1.77 MHz, the Model I wasn’t thought to be fast enough to handle the increased demand of double-density disk transfers.
an interrupt driven disk controller. The Model I disk controller used polled I/O, which added overhead as the disk routines queried the disk controller for every byte.

But the Percom Doubler required neither of those things. It accomplished double-density instead using some very clever hardware and software techniques.
Installation

The Doubler installed inside of the Expansion Interface, in much the same fashion as the Percom Separator. It required no soldering or trace cutting, just removing the floppy disk controller (a Western Digital 1771), plugging it into the Doubler, and then plugging the Doubler into the empty socket.
Software

The Doubler came bundled with DBLDOS, a version of TRSDOS modified by Jim Stutsman to support double-density. Percom also sold OS-80 (formerly known as MICRODOS) which could work with the Doubler. With the notable exception of Model I TRSDOS, Percom Doubler support was soon added to almost all TRS‑80 operating systems.

The DBLDOS double-density routines written by Jim Stutsman are impressive. If you calculate the number of cycles required by a double-density disk transfer, it becomes clear that a Model I ought to be too slow for it to work. What Jim Stutsman did was to turn the approach of the routines around so that he gained just enough cycles to make it possible. The approach he used was copied (sometimes byte for byte!) by other operating systems.
The Doubler II

After it was released, a small percentage of the Percom Doublers developed problems formatting the higher-numbered tracks on disks. In May 1981, Percom introduced the Doubler II, a redesigned version which featured a “ROM-programmed digital phased locked loop data separator.” The price for the Doubler II was reduced to $169.95 and Percom offered a $30 upgrade kit for purchasers of the original Doubler.

According to Harold Mauch, the new version was “more tolerant of differences from diskette to diskette and drive to drive” and also provided “immunity to performance degradation caused by circuit component aging.” He also said that “a Doubler II will operate just as reliably two years after it is installed as it will two days after installation.” The fact that many are still working nearly 30 years after they were installed bears out that assertion.

http://www.trs-80.org/percom-doubler/
01-04-2014, 06:46 AM #2
bohica Member
Posts:1,575 Threads:210 Joined:Feb 2011
I thought my zip Drive was the Cat's meow.
100 MB on a single disc, (portable).

I'm in my seventh decade on Earth. I know things.
01-04-2014, 06:52 AM #3
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
If you had walked into Percom with a zip drive we would have sworn you was a space alien.
chuckle.gif
01-04-2014, 09:08 AM #4
Softy Anonymous Kritter
 
Hi Vain5,

Z80s???,,,6800s???,,,

Digital is the child's play,,,

do analog,,,

Analog is magic...

yay.gif

(:X
01-04-2014, 10:19 AM #5
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
Analog is Black Magic Softy.

The real trick is to make a digital circuit an analog circuit. You get the stability of the digital and the resolution of the analog.

My first computer was analog.

cheers.gif
01-04-2014, 10:23 AM #6
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
Vain5? Ok you got me. I will do my best to be better.

coffeetime.gif
01-04-2014, 10:42 AM #7
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
OK OK analog...I once built an FM crystal set. No batteries, 12" antenna one diode. Could pick up 3 stations.

I designed a sensor for an oil rig that was wireless. It couldn't be magnetic. It had a giant ferrite inside a pipe full of crude oil.

I turned a TV set into an oscilloscope.

I built dozens of short wave receivers. Regenerative, Super regenerative, Superheterodyne.
Tube and transistor models.

I did a Kisser HUD for Bell for an Apache Helicopter simulator.

Slow Scan TV!

cheers.gif
01-04-2014, 11:07 AM #8
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
I did a disk copier. It sucked up an entire track, corrected the positions of every pulse, removed all the write splices and put it back on the track with new write precompensation on the next revolution. It copied any system with any format short of a laser blast in the track.

coffeetime.gif

Analog is labor intense. z80s and 6800s are analog. Making them work was difficult. They were very noisy. And they had idiot tendencies. State Machines, thats where analog and digital come together. The data separator I designed for the Doubler II is faster than any PC of today. I read data, did the math, corrected the clk, in 62.5 nanoseconds using LS and a TTL ROM that was turned off 75% of the 62.5 nanoseconds to save power.

coffeetime.gif
01-04-2014, 06:17 PM #9
Softy Anonymous Kritter
 
(01-04-2014, 10:23 AM)Wayne5 Wrote:  Vain5? Ok you got me. I will do my best to be better.

coffeetime.gif

You started it,,,

not sure why,,,

but there it is...

(:X
01-04-2014, 06:20 PM #10
Softy Anonymous Kritter
 
(01-04-2014, 10:42 AM)Wayne5 Wrote:  OK OK analog...I once built an FM crystal set. No batteries, 12" antenna one diode. Could pick up 3 stations.

I designed a sensor for an oil rig that was wireless. It couldn't be magnetic. It had a giant ferrite inside a pipe full of crude oil.

I turned a TV set into an oscilloscope.

I built dozens of short wave receivers. Regenerative, Super regenerative, Superheterodyne.
Tube and transistor models.

I did a Kisser HUD for Bell for an Apache Helicopter simulator.

Slow Scan TV!

cheers.gif

AM crystal set,,,Amplitude Modulation,,,FM is Frequency Modulation,,,

Regenerative receivers are a pain to tune...

(:X
01-04-2014, 09:02 PM #11
Wayne5 Member
Posts:615 Threads:61 Joined:Nov 2013
True, I detected and demodulated an FM station without external power using one diode.
cheers.gif
01-04-2014, 10:47 PM #12
Softy Anonymous Kritter
 
you said crystal set,,,those were usually AM,,,

AM detectors cannot demodulate FM and PM signals because both have a constant amplitude. However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.

but ok,,,FM,,,you can do it with a digital gate as well,,,

This detection process can also be accomplished by combining, in an exclusive-OR (XOR) logic gate, the original FM signal and a square wave whose frequency equals the FM signal's center frequency. The XOR gate produces an output pulse whose duration equals the difference between the times at which the square wave and the received FM signal pass through zero volts. As the FM signal's frequency varies from its unmodulated center frequency (which is also the frequency of the square wave), the output pulses from the XOR gate become longer or shorter. (In essence, this quadrature detector converts an FM signal into a pulse-width modulated (PWM) signal.) When these pulses are filtered, the filter's output rises as the pulses grow longer and its output falls as the pulses grow shorter. In this way, one recovers the original signal that was used to modulate the FM carrier.

http://en.wikipedia.org/wiki/Detector_%28radio%29

knew you would want a link,,,

anyway,,,excitement city...

(:X



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