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Hardware Reviews

Editorial Review [13 May 2003]

DiSEqC 1.2 motors - Mini group test (Page 1 of 4)


Introduction

The reason for buying a motorized setup is quite simple - there are hundreds of satellites in the sky, with Europe alone keeping over 70 of world's active satellites in geostationary orbit with a main purpose of bouncing TV signals to our households. Satellite enthusiasts might as well want to take a peek at them too, right?

Any dish driving device, has two main tasks to fulfil:
First is the ability to move from point to point on the Clarke belt, where the Geostationary satellites are located, in a fairly consistent and reproducible way. This task, in a manner of speaking, is easy and obvious.

Its second duty would be stop at any target destination with high precision. Our dish has to be able to pinpoint objects in the firmament, roughly 22,000 miles away with an error margin of less than 1 degree, and the bigger your dish the more accurately you need to be able to position it.

All the above has to be done despite weather conditions and natural forces like wind and exposure of metal parts to extreme temperatures. The smaller the inaccuracies, the better. Obviously the motor unit has to be cheap enough to be even considered as an option by the buyer while still remaining profitable for the manufacturer. Take that into consideration and you will quickly understand why most makers avoid at all costs manufacturing rotors for dishes bigger than 1.20cm or 12kg in weight. As for the customer's needs, it didn't take long before the industry realised the two main dos and don'ts of the whole H-H affair, with the most important being the cost and the noise. The sound issue won't bother owners of detached houses too much, but if you happen to share your walls with your neighbours you will always find someone upset enough by the aesthetics of your satellite setup to not be willing to put up with additional noise of your dish browsing around in search of new channels. Luckily years of experience have done the required magic and the manufacturers have come up with most of the possible solutions, including slowing down the rotation by using different gearing, or using nylon/nylon-coated parts instead of metal to hush the grinding noises a little. All that helped solve the problem, and spare our sensitive neighbour's ears a lot, it has to be said. I brought this test to extremes providing you with samples of what every tested motor sounds like just to give you an idea of eventual impact of your hobby on your relationships inside local community (smile).

What is DiSEqC and how does it work?

After the satellite boom in 80s and 90s many Asian and European manufacturers came up with custom mounts pushed and pulled by mechanical actuators. Besides being quite sturdy and strong-armed this solution to every day TV viewer has proven to have just as many disadvantages as favours. Exposed moving parts were prone to wearing out, the construction itself was often hard to adjust and tune for non-professionals and most of all, the setup required multiple cabling from a stand alone or integrated positioner laced outside buildings and windows.

By 1997 the German satellite giant Eutelsat decided to design a simple protocol whereby rotor devices could be operated via set of commands forwarded by the receiver through the same cable used for feeding signal from the LNB. Eutelsat already had a Digital Satellite Equipment Control protocol version 1.0 in place, allowing receivers to switch between up to four dishes or signal feeders, and revision 1.1 for scaling DiSEqC 1.0 by four times. Version 1.2 of the protocol introduced a few new commands required specifically for the purpose of moving the dish across the horizon.

The idea of using a single cable to power both rotor and LNB wasn't new - Nokia had already similar protocol known as Vsec implemented in their products and few other manufacturers worked on own proprietary systems, often, like in case of Irte Omnisat, much more advanced and scalable. However, thanks to impact of the name and good PR Eutelsat's system was a sure winner, and completely free to implement by manufacturers at that.

But how is it done?

Well, let's get back to last decade and look at the first digital receivers - what features did we have available? There was a 14 and 18V supply to LNB, to receive vertical and horizontal transponders, and a 22kHz tone to switch between bands.

A DiSEqC 1.2 positioner can operate a motor by inserting nanosecond gaps for few milliseconds into the existing 22 kHz tone. It is all very clever. Each message is followed by 6 millisecond silence so both the sender and receiver (master and slave) understand where one command ends and another starts. But there is absolutely no reason for me to get into details and graphs here, if you want to know more about the timings and insertion of the gaps simply refer to protocol manuals available for free from Eutelsat website.

The structure of the commands sent between the positioner and the motor is quite simple. They all consist of three parts:

A [Framing byte] an [Address byte] and a [Command byte]

The command byte may be accompanied by additional data bytes.

If we step outside the boring manuals and uninspiring, strict boundaries for a second, perhaps I can explain it better. Imagine that the [Framing byte] is equivalent to an explicit introduction so to speak - similar to "Listen mate" in common speech - and so variations of this prologue can be thought of as:

"Listen carefully mate and keep quiet for a sec" (E0 in hex)
"Listen mate" (implying " and feel free to tell me what you think") (E1) and
"Listen to me mate and I really need your answer"(E2).

The [Address byte] is to define the "mate" we are talking to - e.g. a motor driving from East to West would be codenamed 31, an elevation motor driving up and down, or controlling tilt has handle 32, anything/everything on the line is referred to as 30. There are other possibilities too - "Yo, skew controller" is 21, "polarizer, my dude" is 20. These however are rarely used.

Finally the [Command byte] is the orders we give:
60 Halt - Stop moving
63 Limits Off Disable Limits
66 Limit E Set East Limit (& Enable recommended)
67 Limit W Set West Limit (& Enable recommended)
68 Drive East Drive Motor East (with optional timeout/steps)
69 Drive West Drive Motor West (with optional timeout/steps)
6A Store nn Store Satellite Position & Enable Limits
6B Goto nn Drive Motor to Satellite Position nn

Plus few extra commands, not mandatory, but defined:

64 PosStat Read Positioner Status Register
6E Goto x.x Drive Motor to Angular Position (degrees)
6F Set Positions (Re-) Calculate Satellite Positions

To get our unit to move our dish to position 2 stored in the motor's memory we would say "Listen motor, mate, move to 2, would you?" or E1 31 6B 02 in hex. Simple.

Click to enlarge
If manufacturers wanted to implement all DiSEqC 1.2 commands the menus could be long...

But let's imagine we don't have anything stored in motor's memory at all and want to watch something on Sirius. We can say "Listen mate, move that motor east" (E1 31 68 (value)). But how do we tell the motor to stop or how far to go? The move command has the extra data byte for us to describe the movement. 00 would be "drive that dish till I tell you to stop". Hex values between 01 and 7F, or 1 and 127 tell the motor to drive x amount of seconds "drive that dish for 5 seconds". Negative hex values between 255-1 and 255-126 (80h' to FFh') tell motor how many steps to move "nudge that dish 5 steps". So to find our Sirius, we look at signal strength meter and say at least "Listen mate, move that motor till I tell you to stop" (e1 31 6b 00) and when the signal scale rise we shout "Listen carefully everyone and keep quiet for a sec... stop now!" (e0 30 60). We can then tell motor to remember where we are and store it as numbered position "Listen everyone, and keep quiet for a sec - remember the current point as position two" (e0 30 6A 02).

Technically, because of backward compatibility, the specification demands DiSEqC 1.2 equipment should also support the mandatory DiSEqC 1.0 and 1.1 commands. That would extend the list by

58 Write Freq Write channel frequency
38 Write N0 Write to Port group 0 (Committed switches)
39 Write N1 Write to Port group 1 (Uncommitted switches)
00 Reset Reset DiSEqC microcontroller

but good luck with finding any receiver that has those working while motorized options are enabled.


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