[chbot] The X-plane multiplex problem

Richard Jones richard.jones.1952 at gmail.com
Tue May 15 14:38:26 BST 2018

In response to Marks comment about this must have all been done before this
Stuff article explains how Pacific Simulators started out with three guys
in a garage...


We might be able to persuade one of them to come along for a chat and
describe how all the interfacing is done. I used to work with Stuart Whelan
at Tait and he came along to the robotics group back in the days when were
involved with youngsters at Science Alive.


On Tue, May 15, 2018 at 10:01 PM, Mark Atherton <markaren1 at xtra.co.nz>

> Hey Synco,
> Any chance of a quick tutorial how X-Plane processes incoming UDP packets,
> and assigns associated bytes/bit-positions to each function.
> Getting some kind of spec. from Paul (or help write one) would be very
> useful to determine cable loom lengths, number of inputs, types etc. For
> each control: Push Button, On/Off, Rotary (number of positions) ,or
> Potentiometer (and resolution), and max scan time.
> He may have been using a 4 position switch with 4 wires, when it may
> actually be an encoded 2-bit problem. Not sure if full size, low cost
> rotary binary outputs switches are readily available - hex coded switches
> are obviously too small...  There was mention of an R-2R ladder on the
> switch and an ADC - this seems a bit over the top (?). How about using a 1
> of 4 rotary switch, and a small diode matrix (4 diodes + 4 pull down
> resistors) to encode to binary - there are obviously larger bit-savings
> with 8 and 10 positions switches.
> Not personally a fan of carrying I2C any great distance due to noise
> immunity issues (or lack thereof), but screened twin cable (and associated
> unwanted capacitance) with low clock rate (10kHz ?) and careful grounding
> may be OK though.
> R-PI seems over the top to convert bits into UDP, but I guess they are
> readily available, well supported, and low cost, so why the hell not...
> Need to see if Paul has joined the group email (Andrew, please mail me
> Paul's email address off-list).
> -Mark
> PS can't help but think that someone may have been through all this
> already ...
> On 15/05/2018 6:45 PM, Synco Reynders wrote:
> Hi Paul (from Oxford) + Guys,
>    From what I could gauge Paul was after a simple solution that he could
> understand, build and maintain. So my idea would be to point him in the
> direction of what to buy and how to wire it up, rather then designing a
> custom PCB design.
> So following is an outline of the building blocks I'd use for my solution:
> --------
>          Ethernet/UDP                i2c + power
> X-Plane <============> RaspberryPi <----+--> Panel1 <--> PanelX...
>                                         |
>                                         +--> Panel2 <--> PanelX...
>                                         :
> * Each panel should be self-contained, and have a common connector (think
> ethernet connector + cable is good idea)
> * They can be in a combination of daisy-chaining or star-configuration, to
> suit panel layout.
> A search for "i2c" on TradeMe is probably a good starting point for
> available I2C modules... eg.
> https://www.trademe.co.nz/electronics-photography/other-
> electronics/electronic-components/other/auction-1627424038.htm?rsqid=
> a42479ffc5a54d57a2f2d3102e0a888e
> https://www.trademe.co.nz/electronics-photography/other-
> electronics/electronic-components/other/auction-1628302081.htm?rsqid=
> a42479ffc5a54d57a2f2d3102e0a888e
> https://www.trademe.co.nz/electronics-photography/other-
> electronics/electronic-components/other/auction-1632177890.htm?rsqid=
> a42479ffc5a54d57a2f2d3102e0a888e
> https://www.trademe.co.nz/electronics-photography/other-
> electronics/electronic-components/other/auction-1628152427.htm?rsqid=
> a42479ffc5a54d57a2f2d3102e0a888e
>  * Plan what I2C module are needed and list the used addresses
>  * the RPI / linux has good I2C diagnostics. (apt install i2c-tools)
>  * https://github.com/ControlEverythingCom/Raspberry-Pi-I2C-Python
>  * Run Python on the RPI to do the polling of the I2C modules (switches
> and output lamps etc) and sending the associated UDP packet to X-Plane
>  * Python to X-Plane via UDP (https://github.com/luizcantoni/x-pi)
>  * Here is the list of XPlane parameters that can be read/changed:
> (suspect most are in sim/cockpit2/...)
> http://www.xsquawkbox.net/xpsdk/docs/DataRefs.html
>  * UDP protocol for reference (http://www.nuclearprojects.
> com/xplane/xplaneref.html)
> Might make a mini instrument panel for myself, :-)
> Hope this helps,
> /s
> On 15 May 2018 at 16:09, Trevor Wignall <zl3adz at gmail.com> wrote:
>> Paul from Oxford expressed interest in ways of connecting multiple inputs
>> (switches and potentiometers) to the likes of an Arduino, rather than
>> having to use multiple Arduinos.
>> This is actually a common and easily solved problem. In fact there are
>> multiple solutions, using different approaches with different advantages or
>> disadvantages, using multiplexers, I2C and serial expanders. There are a
>> number of useful integrated circuits (ICs) which could be useful, including
>> members of the 4000 series of CMOS ICs (refer to
>> https://en.wikipedia.org/wiki/List_of_4000_series_integrated_circuits ).
>> Common ICs include the 4051 multiplexer (also known as a one of eight
>> selector), the 4094 serial to parallel shift register, and the 4021
>> parallel to serial shift register. All three of these ICs will run off
>> 5Volts and are cheap, readily obtainable, 16 pin devices.
>> The 4051 is useful for allowing up to 8 digital or analog signals to be
>> scanned using just one digital or analog input of the Arduino, with 3
>> digital output lines from the Arduino being used to select which of the 8
>> inputs to read. It can also be used to select one of up to 8 output lines
>> to drive - more on this later. The same three address lines can be used to
>> drive multiple 4051 selectors.
>> The 4021 is useful for allowing up to 8 digital signals to be read in
>> turn using just one digital input of the Arduino, but requires only two
>> output lines from the Arduino - a "Clock" line and a "Strobe" or "Load" or
>> "Reset" line, which is used to restart the scan process. Multiple 4021s can
>> be wired in series so the same two output and one input lines can be used
>> to scan 16, 24, 32,... digital signals.
>> The 4094 is used to drive up to 8 digital outputs (such as indicator
>> LEDs) using three digital outputs from the Arduino - a "Data" line, a
>> "Clock" line and a "Strobe" or "Load" line. Again multiple 4094s can be
>> wired in series so the same three outputs can be used to drive 16, 24,
>> 32,... digital outputs.
>> But wait - there's less!
>> The same "Clock" and "Load" or "Strobe" lines can be used to drive both
>> 4094 and 4021 shift registers at the same time.
>> A final trick - multiple switches can be scanned by wiring them into a
>> matrix such as an 8x8 array or an 8x16 array. If two switches on the same
>> row or column can be closed at the same time, diodes can be used to allow
>> the sense of each switch to be determines without "Phantom keys". There is
>> no reason an 8x16 array cannot be scanned using a 4094 to drive a 4051
>> which selects one of 8 rows, and a pair of 4021s to read up to 16 switches
>> on that row - or the 12 individual switches of a 12 position rotary switch.
>> And the same outputs of the 4094 can be used to drive the address inputs
>> of a second 4051 which can scan 8 analog inputs. The remaining 5 outputs of
>> the 4094 can be used to drive indicator LEDs and a second 4094 can be added
>> to drive another 8 LEDs if necessary All of this can be achieved with about
>> 5 ports of the Arduino - an analog input, a digital input and three digital
>> outputs including a Clock and a Strobe which can also be used to drive
>> other circuitry.
>> There are other approaches, but this will give an idea of what sort of
>> things can be done.
>> Cheers
>> Trevor
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