Redundant receivers

[CrashGuard]

I was under the impression that Frsky receivers could be connected together using the Fport or S.port (Bus/Port confusion) for redundancy. I hope it does not require a RB-25. I don't have room for that. What is required to do this?

 

[MikeB]

You use SBUS for this. You need a receiver that supports SBUS IN, and another that has SBUS OUT, just connect the OUT to the IN.
Bind both receivers, but disable telemetry on the one using SBUS OUT.

Mike

 

[Wayne]

Depending on what RF version you are running, so if using an Archer receiver on ACCESS, pretty sure you can keep the telemetry feed on both receivers. Someone better confirm this.. LOL

 

[CrashGuard]

I want to get away from 900 MHz telemetry because it is messing with the servos. I still want telemetry though but instead coming from 2.4 GHz and redundancy with two frequencies. The problem is getting the RM9 to talk to the TW SR10 for redundant control.

I'm using a X18s with a TW Module on the back. The TW SR10 has S.port/F.Bus/SBUS_OUT selection to any of 10 pins . The RM9-MM has telemetry S.Port/F.port selection with connection wire and SBUS_OUT with connection wire. I want telemetry from the TW SR10 and receive redundancy with 2.4 GHz and 900MHz. Is this possible without extra hardware?

 

[MikeB]

If telemetry is enabled on more than one receiver, then I believe they will both send at the same time causing the Tx to get garbled data.
I can't see anything in a normal (ACCESS), transmitted packet that specifies a specific receiver should respond.

Mike

 

[CrashGuard]

I believe having both receivers transmitting telemetry would be unnecessary and confounding to the transmitter. So one needs to ensure only one is sending. The on off switch is in each receiver's option list. But this is a side issue. My main problem is getting the receiver redundancy thing working.

 

[Wayne]

Maybe I am wrong - based on my failing memory, but thought that was one of the improvements when they went to ACCESS, ability for the tx to deal with something like 3 telemetry streams.

Yep confirmed:
Key Telemetry & Performance Features
Simultaneous Multi-Receiver Telemetry: Unlike the older ACCST protocol, which only supported telemetry from one receiver at a time (unless using a redundancy bus), ACCESS allows all three bound receivers to transmit telemetry back to the radio.
Automatic Switching: If the primary receiver loses its link, the system automatically switches to the next available receiver's telemetry stream without losing data or triggering a warning.
Increased Data Rate: ACCESS offers a 60% faster telemetry downstream rate compared to the previous ACCST protocol.

 

[CrashGuard]

It would be nice if the TD line of receivers had a selection for which frequency band telemetry came from and we could call it a day. But they do not. Just on and off or <25 mW.

 

[MikeB]

My understanding is only one Rx sends telemetry. If the "primary" Rx loses the RF link, then the next Rx will start to send telemetry. I'm not sure how this is managed, but it may be controlled by linking the SPort signals from the receivers. Only one receiver will control the SPort at a time. I assume the "secondary" Rx will see the SPort go idle for a time and so take over. The telemetry packets do include the index of the receiver that sent them.

For TD, the Tx definitely receives separate data for RSSI, RxBatt and VFR for each frequency band.

Mike

 

[CrashGuard]

Back to focus in the title... How to connect two receivers. Specifically, the TW-SR10 to the RM9-MM. It is not working for me. When I bench test the configuration by turning off the external TW Tx Module the SR10 goes red and the RM9 is still green but the attached servos stop working. The SR10 should get input from the RM9 is what I expect. There is no fail-over. Perhaps something under options, custom fail-safe. I've never setup a redundant system before. Any help would be greatly appreciated.

 

[MikeB]

You may have the connections correct, but please double check. The TW-SR10 already has 2.4G/900M redundancy as it uses both frequencies. If you want telemetry on 2.4G then this should be your primary receiver with the servos connected to it. I think you will still have some telemetry sent over the 900M link. The R9-MM is then the secondary receiver.
Use the SBUS OUT of the R9-MM (not the SPort). Disable telemetry on this receiver.
Configure port 1 of the TW-SR10 to be SBUS IN.
Connect the SBUS OUT of the R9-MM to the SBUS in of the TW-SR10.
Test to see if servos continue to work when the the external module is powered off. Note that an easy way to test is to just change the RxNum being used by the external module, quick and easy to disable/enable the TW-SR10 receiver.

If you are wanting to use the R9-MM as the primary receiver, I suggest configuring as above first an check that all works.

I've done some testing with two, ACCESS receivers, currently in slots 1 and 3 of the Tx. Initially receiver 1 sends telemetry and polls the SPort. receiver 3 does neither. If I power receiver 1 off, then after a delay of 0.84 seconds, receiver 3 starts sending telemetry and polling the SPort. Restoring power to receiver 1 still leaves receiver 3 sending telemetry and polling the SPort.
If I then power receiver 3 off, then receiver 1 starts sending telemetry and polling the SPort, again after a delay of 0.84 seconds.
It seems the receiver not sending telemetry is listening to the one that is sending and times out if nothing is sent for 0.84 seconds, thus causing it to start sending telemetry.

Mike

 

[Wayne]

The R9 stuff is getting pretty old, I do not know how well it works with the newer stuff, heck never was a big fan of the R9 units, FrSky learned a lot during that period and the R9 went through a lot of changes.

TW is a dual 2.4ghz with some Lora protocols, no 900mhz on that one. Mike was thinking of the TD family. I'd think the TW and TD should be happy to work with one another, but I have not tested that.

To summarize, the R9 family is an older generation, I don't know that Frsky has made any attempts to keep it up to date. Can you still buy R9 receivers? We stopped offering them years ago. I was never a fan of them.

 

[CrashGuard]

Okay, i got it figured out ! ! ! The TW_SR10 dose work flawlessly and with the R9M receiver. The TW_SR10 must have hardware limitations in that SBUS_IN is only available on PIN... set One. It does not show in the Ethos drop-down menu for Outputs 2-10. I made the change to direct the redundant pin of SBUS_OUT (R9M) to... (wait for it) >SBUS_IN Pin set ONE< of the TW_SR10. VIOLA. I hope that is clear. I have the R9M telemetry set to off and the TW_SR10 delivers the telemetry at 2.4GHz. Now with NO 900 MHz telemetry, it's turned off, there is NO JITTER on the servos. Telemetry only comes from 2.4 GHz.

The 900 MHz is only a last ditch effort to regain control. I have witnessed ~900 MHz transmissions track power lines at 10's to about 300' feet or more orthogonal to the lines giving 5x+ further distance than expected. It also penetrates the moist canopy better than your microwave oven. So, that is why this frequency band can be useful. Perhaps that is why 850 MHz is the backbone of cell phone Tx.

Frsky sells so many different R9M receivers to this day (2026). It's even confusing to choose which Frsky firmware to select from there are so many. Knowing when and how to use this 900 MHz band is the key. Combining TW receivers with R9M receivers you get the best possible reception. You get LoRa, what ELSR uses, and FSK. Perhaps, SubSpace communication using Quantum tech is the next thing. LOL

Cheers