Need help getting response from RS485 Slave

PeterH,
did you really fail to read and understand my apology.

astrofrostbyte
The section below the code block is probably the most significant part of the code.
as wrote:

The following code is where i beleive the response would arrive.
i have tried adding lines of code to read a particular buffer location ie Rxdata = (buf [1]);
and then do a println to see content but nothing happens.
any suggestions would be greatly appreciated :-

the lines

// receive response  
  byte buf [20];
  byte received = recvMsg (fAvailable, fRead, buf, sizeof buf);
 
  digitalWrite (LED_PIN, received == 0);  // turn on LED if error  
  
  // only send once per successful change
  if (received)
{
    if (buf

    != channel)

     return;  // not my device
      
    if (buf [1] != 2)
      return;  // unknown command
    
byte msg [] = {
       0,  // device 0 (master)
       data,  // turn light on command received
       //data

are what happens when the slave device sends the acknowledge.

i sent the entire code because PaulS complains if i dont.