Controlling Serial Devices [CIS Example]

In this example, we are going to show how to interact with a hypothetical thermostat via a serial (RS232) interface.

CONTROL

The format of the control string is :
Qa,z,n,s<cr>
Where :
'a' is the board address
'z' is the selected zone
'n' is the desired ON/OFF state (OFF = 0, ON = 1)
's' is the desired setpoint in degrees C
<cr> = Carriage Return (ASCII #13)

For example, the command "Q1,2,1,30<cr>" would set board 1, zone 2 to ON at 30C.

The user can set the set point via a System IO variable (called "Set Point"). We want to write some code to transmit the new set point each time it changes. An example of code to do this is :

Code:

{ variables section }
s : string;
LastSetPoint : integer;

{ initialisation section }
OpenSerial(2, 2, 9600, 8, 1, 0, 0);
LastSetPoint := 0;

{ module }
{ send new set point if it has changed }
if (GetIntSystemIO("Set Point") <> LastSetPoint) then
begin
  { prepare the control string }
  format(s, 'Q1,2,1,',GetIntSystemIO("Set Point"):0, #13);

  { send the string }
  WriteSerial(2, s);

  { reset the LastSetPoint variable }
  LastSetPoint := GetIntSystemIO("Set Point");
end;

REPLY

The format of the reply string from the thermostat is :
Ra,z,n,s,t<cr>
Where :
'a','z','n','s' are the current address, zone, on/off and set point as above
't' is the current zone temperature

For example, the reply "R1,2,1,30,20<cr>" would mean than board 1, zone 2 is on, with a set point of 30 degrees and a current temperature of 20 degrees.

We want to extract the current zone temperature from the reply and display it on the screen. The temperature is the last number in the list, so the process to extract the number is as follows :
1. Find the last comma in the reply string
2. Copy everything after the comma to another string
3. Extract the number from this string
4. Display this number

Some code to do this is :

Code:

{ variables section }
s, TemperatureString : string;
i, Temperature, LastCommaPosition : integer;

{ module }
ReadSerial(2, s, #13);

if s <> '' then
begin
  { find the position of the last comma in the string }
  for i := 1 to length(s) do
  begin
    if s[i] = ',' then
      LastCommaPosition := i;
  end;

  { copy the text from the character after the last comma }
  Copy(TemperatureString, s, LastCommaPosition + 1, 3);

  { extract the temperature from the string }
  Temperature := StringToInt(TemperatureString);

  { display the data }
  format(s, 'Current Temp = ', Temperature:3, 'C');
  TextPos(300, 100);
  DrawText(s);
end;

 

Q

hello darren,
what does the q stand for and is it required to communicate to the thermostat (or whatever)
cheers justinw

 

The "q" doesn't necessarily stand for anything in particular. This is a purely hypothetical case to demonstrate how to build up and send serial strings and how to receive serial strings and pull out the relevant bits.

 

Is this the standard system used by a/c rs232 devices? I have a module I am installing that uses the exact same syntax - (accent air system interface module).

Your example code will be invaluable when it comes time to code for it.


Cheers.

 

This is not based on any particular standard. You will need to ensure that the protocol used by your system is identical before using the code.