Serial data transmission input-output delay with Raspberry Pi

Question

The Goal:

Drive several servo/RC motors wireless from one pi to another pi.
In essence, I want to build a RC remote control using a pi, with a second pion the receiver end.

Now I have a serial transmission over a RF link module that is stable and has very few corrupt entries. The serial transmission has a max baud rate of 4800 due to the RF link module.

Problem:

It seems there is a 2-4 seconds difference between the transmitter pi printing values and the receiver pi printing values. I cannot figure out where and why this delay comes from and why is it so large. Please note that the signal on the receiving pi is exactly the same data that is sent by the transmitter pi, but 2-4 seconds later. EVEN when I bypassed the the transmitter/receiver module and connected the Tx and Rx pins with a jumper wire the same delay was seen.

What is causing the data on the receiving Pi to be decoded so much later? I have pasted in the code below.

---------- Tx Pi -----------------

import serial
import struct

ser = serial.Serial("/dev/ttyAMA0")
ser.baudrate = 4800

iCount = 0
bProgramLoop = True
while (bProgramLoop == True):

    iOne = iCount
    if (iOne == 100):
        iOne = 0
        iTwo += 1
        iCount = 0
    if (iTwo == 100):
        iTwo = 0
        iThree += 1
    if (iThree == 100):
        iThree = 0
        iFour += 1
    if (iFour == 100):
        iFour = 0
        iFive += 1
    if (iFive == 100):
        iFive = 0

    lData = [iOne,iTwo,iThree,iFour,iFive] # i have done it like this so that I can track in real time where the transmitter is and receiver is with respect to real time changes on the transmitter side
    sData = struct.pack('5B',*lData)
    ser.write(sData)
    print sData

    iCount += 1

-------------- Rx Pi -----------------

import serial
import struct

ser = serial.Serial("/dev/ttyAMA0")
ser.baudrate = 4800

bProgramLoop = True
while (bProgramLoop == True):
    sSerialRead = ser.read(5)
    tData = struct.unpack('5B',sSerialRead)
    print tData

The time difference between when the Tx Pi prints the string sData and the Rx Pi prints the touple tData is somewhere between 2-4 seconds. Is the struct.unpack function slow?

I need this time difference to be absolutely minimal. Any ideas?

You should consider using the time module or timeit to find where the issue is. I would guess, that it is in the ser.write or print calls — Tom Myddeltyn
– Tom Myddeltyn, Commented May 20, 2016 at 14:47
Also, isn't the ser.write spewing data asynchronously? How do you verify that the 5 bytes you read are the 5 bytes you wrote? — Tom Myddeltyn
– Tom Myddeltyn, Commented May 20, 2016 at 14:53
Hi busfault. Thanks for your reply. Yeah this is an asynchronous data transmission. The reason I have structured the code the way it is is to verify that what I sent is what I am receiving. Which I am, but just delayed. I will give timeit a try, thanks — Archon
– Archon, Commented May 20, 2016 at 15:00
Hi there. Please don't add answer addendums in your question body, nor add [solved] hacks in the title - we prefer not to do that here. Instead, add a self-answer, and accept it by clicking the tick mark. I've added one for you in this instance. — halfer
– halfer, Commented May 25, 2016 at 20:40

Doddie · Accepted Answer · 2016-05-22 19:41:49Z

1

First, I'm not sure the ser.write() is an async call. If this is using the pySerial library, the docs say that it is a blocking call.

Perhaps try:

...
ser.write(sData)
ser.flush()    # Force the 'send'
print sData
...

Also, your ldata might be easier to populate like so:

lData = [iCount % 100, iCount % 10000, ...etc]

Also, setting a write timeout might help (but I don't think so).

edited May 22, 2016 at 19:41

answered May 22, 2016 at 14:12

Doddie

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2 Comments

Archon Over a year ago

Hi Doddie. Thanks for you reply!! I will give your suggestions a try later tonight when I am home. You didnt seem sure with the method I am using, would you do the same thing differently? How would you code a serial transmission that has little lag and uses as few as possible bits to send over data?

Archon Over a year ago

Hi Doddie. ser.flush is the answer. It lowers the overall sample rate of the mainloop a little but this is not a deal breaker for me. I am getting near instant signal transmissions. Thank you very very much!!!!!!

halfer · Accepted Answer · 2016-05-25 20:38:09Z

0

(Posted on behalf of the OP).

As suggested by Doddie use the ser.flush command just after ser.write. This results in a near instant response on the Rx side. The overall mainloop sample rate has dropped a bit, but for me at least that is not a deal breaker.

answered May 25, 2016 at 20:38

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halfer

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