Radio Signal Latency

Transmitter-to-Receiver latency is a much-cited metric when discussing radio systems.

Andy Kunz, a member of the Spektrum development team, wrote a couple of posts going into great detail about how latency is measured and what it means.  Because RCG posts get hard to search, I'm capturing them here for posterity.

I've moderately edited from these posts and a couple in between (quoting people asking questions).

Thanks Andy!

Using the Spektrum DM9 module with Taranis and Turnigy 9xr

Daedelus66 notes that these settings work when using a DM9 in a Taranis (and presumably other {er,open}{9x,tx} radios):

• polarity set to +, rather than the default - (which works with the Orange module).
• preferable but not required to set PPM to 400u instead of the default 300u. ensures neutral remains at 1.5ms -- if left on 300u it will be significantly shifted and require recentering servos.
• DM9 uses only the three basic pins: signal #1, positive #3, and ground #4. Pins 2 and 5 are unused, so no problem there.
• model match not supported.
• rebind by holding the module button while powering up.

One Minute Guide to DX6i Binding

Here's how to bind a Spektrum DX6i to a receiver.

• Tx power off
• Insert bind plug into Rx bind port.  Polarity doesn't matter
• (optional) Plug a servo into Rx AIL channel.  Not necessary, but you can use it to see if your binding worked.
• Insert ESC plug into any RX channel.  Doesn't matter which one, since we're only concerned about power.
• Rx LED will fast blink.
• Move Tx about 5-6 feet away from Rx
• Pull up Tx Trainer switch.  Turn on Tx.  Screen will show "Bind"; Keep holding Trainer switch
• Rx LED will: stop flashing; resume slow flashing; stop flashing;  Wait a couple more seconds.
• If you have a servo plugged in, it will come to life.  Test by moving AIL channel.
• Release Trainer switch.
• Power off Tx, Rx, remove bind plug.

Two biggest problems:

• Tx and Rx are too close.
• You don't hold the Trainer switch for enough time.

LemonRX Shrink Wrapping

 LemonRX is a new manufacturer of DSM2 (Spektrum) compatible receivers.  They've been getting lots of good press about their performance, build quality, and price.  At 2.5g, they're about as light as can be.

They come as bare boards, so I wanted to toughen them up a bit.  I started out by quickly applying the smallest dab of hot glue to the antennas.

 Next, I cut some 3/4 inch heatshrink tube to a length that covered the exposed electronics and a bit past the antennas.  I cut two slits in the side for the antennas to go through.

Binding Spektrum DSM and Turnigy OrangeRX Receivers

Here's how to bind a DSM{2,X} transmitter and receiver.  This applies to both Spektrum and OrangeRX units.

Safety First: Remove your props.  If you screw something up, your motor could unexpectedly spin up at 100% power.  That's an uninteresting problem to resolve.

Setup

• Remove props.
• Power off Rx, Tx.
• Make sure ESC is plugged into the THR port of the Rx.
• Plug bind plug into Rx bind port.  Polarity does not matter.

Binding

• Power up plane.  The Rx LED should fast blink.
• Move Tx sticks into desired failsafe position.  Most importantly, make sure THR stick is at lowest position.
• Press and hold Tx bind button.
• While keeping Tx bind button depressed, power on Tx.
• While keeping Tx bind button depressed, wait for Rx LED to go solid.
• Release Tx bind button.
• Power off  Rx.
• Power off Tx.
• Remove bind plug from Rx.

Testing

• Power on Tx.
• Power on Rx.
• Controls should be operational Fiddle with sticks and adjust Tx settings appropriately.

Testing Failsafe

• Adjust throttle so that motor is spinning.
• Move sticks so that control surfaces are deflected.
• Power off transmitter.
• Motor should stop spinning.  Depending on your Rx model, the control surfaces will either stay in place or return to the failsafe positions set while binding.
• Power on transmitter.  Stick control should resume.

Range Testing

• Attach the model to the ground.
• Move away 90 feet.
• Press and hold the Tx Bind button.  This puts your Tx into reduced power range test mode.
• Ensure all controls work as expected.

Spektrum Dx6i $5 backlight mod

This uses the Turnigy 9x backlight from Hobby King:

colors available: blue  green  white

It's a bit more complicated than modifying the 9x, because you can't use the supplied wire harness.  Instead you clip the wire harness off, and solder the leads to the board.  Reports indicate it's about a 30 minute job.  I'll allocate an evening to it.

I've got this on my todo list... in the meantime, here's a link to the original article.
Follow to at least page 3, where there's revised instructions.

http://www.hobbyking.com/hobbyking/forum/forum_posts.asp?TID=18137

Hacking the I2C interface of Spektrum DX and AR ?

RC-CAM Forum: "The Spektrum transmitter has a small daughter board in it that contains the Unigen RF module, a voltage regulator, and a host microntroller. The microcontroller is used to measure the PPM signal from the transmitter's encoder and convert it to a three-wire (I2C) communication signal so that it can talk to the Unigen module. ...."