Missing first byte stored in a buffer. Need some help with this.

Don M · 2013-05-01 18:10

I am receiving in some data into a buffer. For some reason I am missing the first byte. It must be the way I have this loop structured but can't figure why it won't work.

I can see the data on the analyzer so I know it's there but somehow my indexing is off by one. The buffer is 32 bytes. The final check byte has 9th bit set so that is the reason for the $1_00 check.

  inx := 0 
  s := serial.rx_time(20)
  if s > -1
    'inx := 0
    chksum := s 
    buffer[inx] := s 
    repeat
      s := serial.rx_time(5)
      if (s & $1_00)
        chksum &=  $0_FF
        if (chksum == (s & $0_FF))
          pollack
          seq++ 
          term.str(string("ID: ",13,10,03))
          quit
      chksum += s
      buffer[inx] := s
      inx ++
  else
    term.str(string("No Response to ID",13,10,03)) 
    seq := 4        

  repeat inx from 0 to 28
    term.tx(byte[@buffer][inx])
    term.tx(32)
  term.tx(13)

Don M · 2013-05-01 18:23

If I add a "term.tx(s)" in a couple places I see on PST what I need but for some reason the buffer misses the first byte.

  if s > -1
    chksum := s 
    buffer[inx] := s
    term.tx(s)     '<<< This shows the first byte. Why isn't it stored in buffer?
    repeat
      s := serial.rx_time(5)
      buffer[inx] := s
      if (s & $1_00)
        chksum &=  $0_FF
        if (chksum == (s & $0_FF))
          pollack
          seq++ 
          term.str(string("ID: ",13,10,03))
          quit
      chksum += s
      buffer[inx] := s
      term.tx(s)      
      inx ++

kwinn · 2013-05-01 18:31

It looks like it should work. Try changing inx := 0 to inx := 1 and increasing the buffer size by one to see if the first byte is lost in the input loop or output loop.

Don M · 2013-05-01 18:38

No that didn't help.

What I don't understand is if I can see the first byte from the first term.tx(s) then why isn't this same byte stored to buffer[idx] when idx starts at 0?

I can do term.tx(byte[@buffer][0]) and I see the second byte that is received not the first. Weird....

jurop · 2013-05-01 18:42

Try changing

...
      chksum += s
      buffer[inx] := s
      inx ++
  else
...

with

... 
       chksum += s
       buffer[++inx] := s
  else

Don M · 2013-05-01 18:46

jurop wrote: »

Try changing

...
      chksum += s
      buffer[inx] := s
      inx ++
  else
...

with

... 
       chksum += s
       buffer[++inx] := s
  else

Nope. Still doesn't show correct first byte from buffer.

Don M · 2013-05-01 19:09

Ok I got it to work.

Seems as though inx was not being advanced after the first byte stored. See below.

  inx := 0 
  s := serial.rx_time(20)
  
  if s > -1
    chksum := s 
    buffer[inx] := s
    inx++               '<<<< Here is where my problem was....
    repeat
      s := serial.rx_time(5)
      if (s & $1_00)
        chksum &=  $0_FF
        if (chksum == (s & $0_FF))
          pollack
          seq++ 
          term.str(string("ID: ",13,10,03))
          quit
      chksum += s
      buffer[inx] := s
      inx++
  else
    term.str(string("No Response to ID",13,10,03)) 
    seq := 4

Don M · 2013-05-01 19:13

This works too...

inx := 0 
  s := serial.rx_time(20)
  
  if s > -1
    chksum := s 
    buffer[inx++] := s
    repeat
      s := serial.rx_time(5)
      if (s & $1_00)
        chksum &=  $0_FF
        if (chksum == (s & $0_FF))
          pollack
          seq++ 
          term.str(string("ID: ",13,10,03))
          quit
      chksum += s
      buffer[inx++] := s
  else
    term.str(string("No Response to ID",13,10,03)) 
    seq := 4

Missing first byte stored in a buffer. Need some help with this.

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