Replacing RS485 link with NRF24L01+

[o.hojvat]

As Pevious ticket: Using NRF24L01+ with pure assembler programing becomes very long,  I open a new one as Torbjørn suggested

This is a description about what I have doing in my proyect and at the end there are some questions.

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My application has a Central unit connected to several Handheld controllers  with RS485 cable.
Bit rate is 62.500 , 9bits

The central unit is sending in secuence a Callbyte for each Handheld address (from1 to 31) and wait about 100 us for a reply, before it will send a Callbyte to the following address
In this cable system a Handheld only changes to Tx mode after a Callbyte with his address is received.

The Central unit has connected two rails for Model Electric trains. (Ferromedelism)

I will add an interfase  with a uP and a NRF24L04p that will handle the RF link and connected to the Central by 485 cable
No modifications at the Central unit hadware or software willbemade.

The the handheld will be re-designated by adding one NRF24L01+, removing the 485 part and modifying the program as needed.

Using the usual star configuration , the intefase will use PRX mode and may be up to 6 handheldwill using the NRF part in PTX mode.

All the 485 packets are 9 bits but the MSB is only for the information about if the byte is an Address (= 1) or a DATA  (= 0)

Only the Central may send Address and Data bytes.   Handheld units only send DATA bytes

In my aproach the interfase unit decodes only the Callbytes and the NRF will need to handle only 8 bits.

There are 3 types of Callbytes:

1) Callbytes with no more bytes in the packet, that allows the matching address Handheld to

send a packet to the central

2) Callbytes with 1 or more DATA with information requested before by one specific handheld.

The regarding address will be in the Callbyte

3) Broadcast Callbytes: The packet will have information that must be sent to all the Handhelds units.

The program for the interfase unit has 6 RX buffers and 6 TX buffers for the USART and 485 , one pair for each PIPE

Data send from a Handheld by NRF is received by interfase and stored in the TX buffer , for case 1)

Data received by USART (from RS485) is stored in Rx buffer and loaded in NRF TXFIFO  for sending it as an ACK PAYLOAD

I will use some time slots system (not defined yet) for each PTX for avoid collitions and to get back  information when available

------------------------- Some quetions here ---------------------

At the moment the matter  1) is working OK   

Each information sent from Handheld is received at interfase side and sent by RS485 when type 1) Callbyte arrives.

For cases 2) and 3)  ,  information from Handheld is received but ACK PAYLOAD is sent some times yes  other times no

uP reset do not helps .  If the NRF supply is disconected and reconected, ACK PAYLOAD begins to work

It may be  working in all the PTX packets or alternately one yes one not.

I have a low cost , 8 channels Logic Analizer and I will investigate this problem by reading STATUS and FIFO_STATUS

I guess the problem is at PRX side, not sending ACK, but I am no sure.

Please send any suggestion about where it may be the problem.

Also how to diferenciate  which side is not working well.

Best Regards, Osvaldo Hojvat

[o.hojvat]

Hi Torbjørn

Thank you very much for your always kind answer. I will take into account all your suggestions.

Now I'm trying to get the synchronization of the PTX timer with the PRX timer working, using the periodic transmission which is necessary anyway.

 Every 128ms PTX sends a packet that starts with F5 and then the value of its PTXtimer, which will always be the same at this moment.

 PRX recognizes F5 code and only loads the value of the difference (PRXtimer - PTXtimer) as ACK payload
.
 PTX receives it and adds that value to the PTX timer with which it remains synchronized.

The total time is 420us and the timers advance every 500us, so there is no problem that they change during the process.

I used transmit the difference in case there is a delay due to ret-ransmissions o other, but I can measure and correct that.
 
This way it seems to work OK, but a few times it initiates and remains oscillating indefinitely.

It is not easy to start the fault.     It is fixed when the PTX sends a normal use packet inserted and the next one with F5 does not update.

 As it is a feedback system and with delays, I tried to correct using half the difference and the oscillation is damped in 1 to 2 seconds.
 
This is acceptable but I want to understand how it works and correct it properly.

 Now I believe that it should not be corrected in all packets.

 I have to analyze well the capture of the 8 channels of the logic analyzer, 4 of the SPI of the PTX and 4 of the PRX.

 I will send the result when I have it solved.

Best Regards, Osvaldo

[ovrebekk]

Hi Osvaldo

o.hojvat said:

 Every 128ms PTX sends a packet that starts with F5 and then the value of its PTXtimer, which will always be the same at this moment.

 PRX recognizes F5 code and only loads the value of the difference (PRXtimer - PTXtimer) as ACK payload
.
 PTX receives it and adds that value to the PTX timer with which it remains synchronized.

You mean to say you send two packets from the PTX? 

One with the F5 command and a second one to pick up the ACK payload?

o.hojvat said:

I used transmit the difference in case there is a delay due to ret-ransmissions o other, but I can measure and correct that.

You can disable retransmission if you want, and do it manually. If you don't get an ACK just restart the process, and use an updated PTXtimer value to ensure that the timing is accurate. 

o.hojvat said:

This way it seems to work OK, but a few times it initiates and remains oscillating indefinitely.

Can you provide more details regarding this oscillation? You mean the timing keeps oscillating between different offsets?

Getting a trace of the SPI bus of hte PTX and PRX makes sense. Then you might be able to spot what the reason for the problem is. For instance it might be caused by packet loss somewhere, which should be easy to spot from the trace. 

o.hojvat said:

 I will send the result when I have it solved.

Sounds like a plan. Just get back to me when you have the traces. 

Best regards
Torbjørn

[o.hojvat]

Hi Torbjørn

The correct procedure to synchronize the timer of the PTX is the following;

 PTX sends a packet

PRX responds with ACK and payload and at that moment loads the value of its timer as payload

 PTX receives ACK but discards the possible payload because it does not have the information recently loaded.

 PTX sends a second packet 128ms after the first one

 PRX responds with ACK and in the payload it has the timer information.     This information corresponds to 128 ms previous, but at that time both timers overflow and return to the same value (+/- 0.5ms in the case of PRX)

PRX gets an ACK payload and place the value received in its timer -> both are synchronized.

 It can be repeated but always transmitting two packets and using the ack payload of the second.

The error was in modifying the time between the two packets sent by PTX, by updating the PTX timer when receiving the first ack.
If the correction was 0 or almost 0, the oscillation was not noticed.

I will send images in next post

Best Regads, Osvaldo

[o.hojvat]

Please tell me how to attach a PDF  file

I am using WindowsXP but I can move to a W10  PC

Regards, Osvaldo

[ovrebekk]

Hi Osvaldo

You should be able to attach any file by drag dropping it onto the message reply window.

The window should turn gray when you drag a file over it, and then you can release it. 

Best regards
Torbjørn