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Is the Losmandy G11 492 Digital Drive a "Classic" Yet?

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#1 Geo.

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Posted 31 May 2021 - 11:24 AM

The Losmandy 492 Digital Drive dates from, wait for it, April 1992. Yup 4/92. Ah, where were those creative marketers in 1992? In the late '80s Celestron commissioned Tangent, which provided the Advanced Astro Master digital setting circles controller to come up with an accurate battery operated tracking system for the upcoming Ultima 8. The new unit was hoped to replace the rather expensive and sort of make shift adaptation of the Vixen DD2 used in the PowerStar mounts. In 1990 the Ultima 8 belatedly got its Tangent system. 

 

By this time plans were afoot for the C11 to be sold as a package with a new German mount being developed by Losmandy (a/k/a Hollywood General Machining). This package was introduced in 1992 as the G11. It was equipped with the 492 system which was an adaptation of the Ultima drive. While the Ultima was built around a closed loop servomotor system an open loop step motor system was selected for the G11. Given the higher torque requirements this is not surprising. The 492 was later adapted for use in the Losmandy GM-8 mount. Until recently still available as the standard drive system for the G11 and GM-8. It is currently not available from Losmandy. 

 

Losmandy 492.jpg

 

So what to do when your GM-8 492 gives up? Well G11 owners who upgrade their drives to a goto system can be a source. But there is a small problem if you are looking for a GM-8 system and there are only G11s available. I was recently faced with is and here's how it was easily overcome with the help of Mike Herman, who hangs out here and at the Google Losmandy group.

 

The major stumbling block between the GM-8 and G11 492s is the that GM-8 has a 180:1 worm and the G11 360:1. Therefore, the step pulse timing has to be different. After talking with Mike the solution seemed inexpensive and fairly easy. All that was required was a 6MHz crystal for the microcontroller's clock and a EPROM flashed with the GM-8's control parameters, which Mike was able to provide. The EPROM is a 27C256 and dare I say this is a Classic memory design? It was a development of the first EPROM invented in 1972. With a capacity of 256K bits it can store 32K bytes. EPROMs are programmed at high voltages (usually 28V) and are erased by shining high energy UV light at them through a window. so it's fairly easy to pick out the 27C256 in the photo of the 492 controller, just look for the window.

 

492-GM8.jpg

 

Fortunately, the EPROM is not soldered to the circuit so removal and replacement is easy. The the G11's 12MHz has to be removed. and it is soldered to the circuit board. I removed it by heating the points under the board and pulled it out. Rather than attempting to reuse the through holes that involves some risk to damaging them and the printed traces they are connected to, I used a surface mounted replacement crystal. This was soldered directly to the legs of its capacitors. And there you have is a GM-8 controller.

 

492-GM8-A.jpg

 

 


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#2 Pete W

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Posted 31 May 2021 - 03:34 PM

George, thanks for the brief tutorial on this conversion.  My GM8 mount is about the same age as yours so I cross my fingers every time I turn it on.


Edited by Pete W, 31 May 2021 - 03:35 PM.


#3 mich_al

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Posted 31 May 2021 - 05:05 PM

Geo, had to check your bio.  Found you are not the millenial I expected to find.  You speak of 1992 like it was before the ice age!  To me it wasn't that long ago and neither are EPROMS with windows.  Long before that I built a burner that programmed 2716's,  your targets much older ancestor.   FWIW it ran on a TRS-80 and I sold the construction article, complete with assembler SW,  to Kilobaud magazine.


Edited by mich_al, 31 May 2021 - 05:12 PM.


#4 John Fitzgerald

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Posted 31 May 2021 - 05:09 PM

It's not a classic.  I bought a brand new G11/492 from OPT in early 2019.  It was manufactured in November of 2018.



#5 BRCoz

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Posted 31 May 2021 - 05:48 PM

Wow brings back memories.  In the 70s the first computer I operated was an IBM360 with 256k of RAM.  The box the RAM was in was the size of a refrigerator.  Now a smart phone has more processing power. 

 

My GM8 is from around 2001 and the G11 with go to version 1 is a few years later. Thinking about buying the G2 for the GM8.  



#6 jkmccarthy

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Posted 31 May 2021 - 05:52 PM

I recently purchased a just-taken-out-of-service Celestron-branded 492 controller + handpaddle + 2 hurst motors and cables from someone who had just upgraded their Celestron "Losmandy G9" mount (purchased together with the Celestron 9.25 SCT) to the a Gemini 2 go-to drive system.  Searching on-line, I found the number of teeth on the Losmandy G9's R.A. worm gear (180) tabulated here alongside numerous other Losmandy and Celestron (and other) equatorial mounts:

 

https://www.siriusim...equiipment.html

 

So I just thought I'd share this link for others' benefit who may be considering the purchase of a used 492 controller removed from a ______ mount.

 

Hope this helps, and thanks George for the history and technical electronics information !

 

          -- Jim

 

P.S.   I'm curious what other mount-specific "control parameters" are burned into the EEPROM on the 492, that could(?) also potentially pose obstacles retrofitting a retired 492 drive controller and motors package onto another GEM, once the "major stumbling block" of adapting to the correct number of R.A. worm gear teeth has been addressed (by ensuring the number of gear teeth in the original [retired] and future [repurposed] mounts match, or by replacing the master clock's crystal oscillator ... to ensure the 492 runs at 6MHz for 180 tooth R.A. worm gears, and 12MHz for 360 tooth R.A. worm gears) ?


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#7 starman876

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Posted 31 May 2021 - 06:54 PM

The losmandy drive system can be programed and installed on other mounts besides losmandy.



#8 blakestree

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Posted 31 May 2021 - 09:06 PM

It's not a classic.  I bought a brand new G11/492 from OPT in early 2019.  It was manufactured in November of 2018.

How does one go about dating a G11/492 combo?



#9 starman876

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Posted 31 May 2021 - 09:27 PM

How does one go about dating a G11/492 combo?

I am  not sure when they started selling the G11 with the 492 drive system, but I was a long time ago maybe a far back as 1992


Edited by starman876, 31 May 2021 - 09:35 PM.

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#10 Geo.

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Posted 01 June 2021 - 09:44 AM

Generally, the ICs on the circuit board have manufacturing date. Usually by week and year. They can give an approximate date of  sale. The circuit board is no help. It's still dated 1992. I don't think it was ever known as the StarTrack controller. Probably too close to Roger Tuthill's StarTrap trademark.

 

The first G11 I had had a 1992 in the serial number and the knurled clutch knobs. The fluted knobs came later and have different threading.

 

The G-9 was just a GM-8 with the G11's 3" wide saddle. At the time it was sold by Celestron, Losmandy sold the GM-8 with a saddle with a 2.5"(?) wide saddle. Probably making the G-9 model for Celestron inspired doing away with the GM-8 specific dovetail design and standardizing all saddles at 3". I don't know what Celestron was thinking, asking for a heavier saddle and then putting the mount on a Synta copy of the Vixen AL-130 tripod. Oddly the adapter Losmandy made for the AL-130 also fit the Celestron standard SCT field tripod.

 

Losmandy.jpg

 

The first mainframe I worked on had 64K bytes of core memory. At that point core was about 1 sq/in per byte. First time I came across a 8GB micro SD card I had to run the numbers. Concluded that I would need a quarter square mile of core to hold the same data as the micro SD. My $80 Motorola cell has about 100X the power of the mainframe that put Neil on the Moon in 1969.

 

This design by Roman Hujer [https://www.thingive...m/thing:4750134] is the latest thing I'm playing with. Roman's PCB design is an Arduino "shield" for the ESP32 based ESP-duino development board. https://github.com/SmartArduino/SZDOITWiKi/wiki/ESP8266---ESPduino-32

 

P1016962.jpg

 

P1016960.jpg

 

The shield provides connections for two step motors and their drivers, a ST-4 port, WiFi, real time clock, audio signal, status LEDs and a 36-19V to 3.3V buck converter. The ESP32 is a 32 bit dual core IoT device that runs at 240MHz.  Its major advantage is that it can be programmed using the Arduino IDE through the USB port and, oh yes, it's cheap. 

 

This is what the IDE reports about its memory. 

 

Sketch uses 340377 bytes (25%) of program storage space. Maximum is 1310720 bytes.
Global variables use 21156 bytes (6%) of dynamic memory, leaving 306524 bytes for local variables. Maximum is 327680 bytes.

 

Bit more than the 492's 32K!

 

 

 


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#11 starman876

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Posted 01 June 2021 - 11:58 AM

I have been playing around with the si tech controller.  

 

 

https://www.siderealtechnology.com/



#12 jkmccarthy

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Posted 01 June 2021 - 01:30 PM

An interview with Scott Losmandy, posted to YouTube in 1997, in which he discusses the evolution of his drive controller systems starting here:  https://www.youtube....pLdFPnU4&t=383s



#13 mich_al

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Posted 02 June 2021 - 03:07 PM

This design by Roman Hujer [https://www.thingive...m/thing:4750134] is the latest thing I'm playing with. Roman's PCB design is an Arduino "shield" for the ESP32 based ESP-duino

 development board. >>https://github.com/SmartArduino/SZDOITWiKi/wiki/ESP8266---ESPduino-32

 

Went there for a look see but didn't find any real content.  Are there schematics or code available ?


Edited by mich_al, 02 June 2021 - 03:08 PM.


#14 blakestree

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Posted 06 June 2021 - 08:06 PM

Generally, the ICs on the circuit board have manufacturing date. Usually by week and year. They can give an approximate date of  sale. The circuit board is no help. It's still dated 1992. I don't think it was ever known as the StarTrack controller. Probably too close to Roger Tuthill's StarTrap trademark.

 

 

Handwritten on my board, on either side of the voltage regulator, is 8 --- 05. If this means August of 2005, it would seem to jive well with my HGM06**** serial number.

 

BTW, is it okay to spot clean the circuit board with rubbing alcohol? A tiny bit of schmutz got in around my power switch.


Edited by blakestree, 06 June 2021 - 10:19 PM.


#15 jkmccarthy

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Posted 07 June 2021 - 11:07 AM

Geo., on 31 May 2021 - PDT 09:24 AM said:

 

[...]  After talking with Mike [Herman] the solution seemed inexpensive and fairly easy. All that was required was a 6MHz crystal for the microcontroller's clock and a EPROM flashed with the GM-8's control parameters, which Mike was able to provide. [...]

 

 

[...] I'm curious what other mount-specific "control parameters" are burned into the EEPROM on the 492, that could(?) also potentially pose obstacles retrofitting a retired 492 drive controller and motors package onto another GEM, once the "major stumbling block" of adapting to the correct number of R.A. worm gear teeth has been addressed (by ensuring the number of gear teeth in the original [retired] and future [repurposed] mounts match, or by replacing the master clock's crystal oscillator ... to ensure the 492 runs at 6MHz for 180 tooth R.A. worm gears, and 12MHz for 360 tooth R.A. worm gears) ?

Does anyone know what other mount-model-specific "control parameters" might be burned into the EPROM in the 492 ?
 

Thanks,

 

       -- Jim


Edited by jkmccarthy, 07 June 2021 - 11:19 AM.


#16 Geo.

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Posted 07 June 2021 - 02:09 PM

The 80C52 has 256 bytes of on-chip RAM, 8K of PROM and can address 64K of external PROM. It's 8-bit rnicrocontroller optimized for control applications. There are 30 GPIO pins. So I have to assume that the 32K PROM would have to contain the programming for most functions. By 1996 the 80C56 had 64K PROM, but looks like no effort was made to take advantage of the new capacity. 

 

The ATmega328P microcontroller has 23 programmable I/O lines: 6 needed to fully control 2 step motors. 

 

● 32K bytes of in-system self-programmable flash program memory

● 1Kbytes EEPROM
● 2Kbytes internal SRAM
● In-system programming by on-chip boot program
● True read-while-write operation

 

Peripheral features:


● Two 8-bit Timer/Counters with separate prescaler and compare mode
● One 16-bit Timer/Counter with separate prescaler, compare mode, and capture mode
● Real time counter with separate oscillator
● Six PWM channels
● 8-channel 10-bit ADC
● Temperature measurement
● Programmable serial USART
● Master/slave SPI serial interface
● Byte-oriented 2-wire serial interface (Phillips I2C compatible)
● On-chip analog comparator

 

Two bucks for the IC or $3 with it on the basic Arduino Nano development board. It should be able to deal with everything the 492 handles. The 6 G/S settings can be set up in if...else statements as can the RATE settings. PEC code is available in the public domain. Just have to pull the code together. The 1987 Compustar managed goto with a limited catalog of objects with the 8051, a close cousin of the 80C52. The big brother of the ATmega328P the Mega2560 already supports goto. 

 

Here we have a step motor, controller and remote human interface. Four bucks. I can't recall if the microprocessor on the controller is a 8050 or 8051 variant, but it can handle driving motor without a separate driver. It has to sink 240mA. I priced the microcontroller at 24 cents for 1 unit. These are the type of commodity processors the auto industry can't get enough of. The IR remote is a clone of a Sony auto CD changer unit.

 

P1016647.jpg

 

 

 

 




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