matlab serialport in s-function error with Undefined function 'write' for input arguments of type 'string'

I have an RS232 communication device and I want to send two different data frames and receive the response in a single step. As far as I know, the serial send block in Simulink's Instrument Control Toolbox doesn't support this functionality. Therefore, I intend to implement this using serialport in an S-Function, but I've encountered some issues. First, here is the MATLAB script I tested, which works correctly.

serialObj = serialport("COM16",115200);
configureTerminator(serialObj,"CR/LF","CR/LF");
flush(serialObj);
dataframe1 = [0x53 0x07 0x01 0x01 0x64 0x60 0x00 0x56 0x45];
dataframe2 = [0x53 0x07 0x02 0x01 0x64 0x60 0x00 0x00 0x45];
write(serialObj,read_node1_pos,"uint8");
response = read(serialObj,13,"uint8");
write(serialObj,read_node2_pos,"uint8");
response = read(serialObj,13,"uint8");
serialObj.delete();

I converted it to an S-Function version, and the code is as follows:

% MySerialSend.m
function [sys,x0,str,ts,simStateCompliance] = GetMotorPos1(t,x,u,flag)
persistent serialObj
switch flag,
  case 0,
    [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes(serialObj);
  case 3,
    sys=mdlOutputs(t,x,u,serialObj);
  case {1,2,4}
    sys=[];
  case 9,
    sys=mdlTerminate(t,x,u,serialObj);
  otherwise
    DAStudio.error('Simulink:blocks:unhandledFlag', num2str(flag));
end

function [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes(serialObj)

sizes = simsizes;

sizes.NumContStates  = 0;
sizes.NumDiscStates  = 0;
sizes.NumOutputs     = 2;
sizes.NumInputs      = 0;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;   % at least one sample time is needed

sys = simsizes(sizes);
x0  = [];
str = [];
ts  = [-1 0];
simStateCompliance = 'UnknownSimState';
 
if isempty(serialObj)
    serialObj = serialport("COM16",115200);
    configureTerminator(serialObj,"CR/LF","CR/LF");
    flush(serialObj);
end

function sys=mdlOutputs(t,x,u,serialObj)

dataframe1 = [0x53 0x07 0x01 0x01 0x64 0x60 0x00 0x56 0x45];
dataframe2 = [0x53 0x07 0x02 0x01 0x64 0x60 0x00 0x00 0x45];
write(serialObj,dataframe1,"uint8");
response = read(serialObj,13,"uint8");
write(serialObj,dataframe2,"uint8");
response = read(serialObj,13,"uint8");
sys = [];

function sys=mdlTerminate(t,x,u,serialObj)
delete(serialObj);
sys = [];

Upon execution, I encountered the following error:

'''

Error:An error occurred while running the simulation and the simulation was terminated

Caused by:

Error in 'RS232_test/S-Function' while executing MATLAB S-function 'MySerialSend', flag = 3 (output), at time 0.0.

Undefined function 'write' for input arguments of type 'string'.

'''

I have searched extensively but couldn't find any similar errors. Could anyone provide some suggestions? If more information is needed, please let me know. Thank you very much!

Answer 1

[Walter Roberson]

You do not initialize serialObj within the same scope as the persistent declaration. You end up passing empty serialObj everywhere. You set it inside of a function but the function does not return it to the level above.

[You-Cheng]

Hi, Walter, thanks for your answer!

After modifying the S-Function to:

% MySerialSend.m
function [sys,x0,str,ts,simStateCompliance] = GetMotorPos1(t,x,u,flag)
persistent serialObj
if isempty(serialObj)
    serialObj = serialport("COM16",115200);
    configureTerminator(serialObj,"CR/LF","CR/LF");
    flush(serialObj);
end
switch flag,
  case 0,
    [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes(serialObj);
  case 3,
    sys=mdlOutputs(t,x,u,serialObj);
  case {1,2,4}
    sys=[];
  case 9,
    sys=mdlTerminate(t,x,u,serialObj);
  otherwise
    DAStudio.error('Simulink:blocks:unhandledFlag', num2str(flag));
end
function [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes(serialObj)
sizes = simsizes;
sizes.NumContStates  = 0;
sizes.NumDiscStates  = 0;
sizes.NumOutputs     = 2;
sizes.NumInputs      = 0;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;   % at least one sample time is needed
sys = simsizes(sizes);
x0  = [];
str = [];
ts  = [-1 0];
simStateCompliance = 'UnknownSimState';
function sys=mdlOutputs(t,x,u,serialObj)
dataframe1 = [0x53 0x07 0x01 0x01 0x64 0x60 0x00 0x56 0x45];
dataframe2 = [0x53 0x07 0x02 0x01 0x64 0x60 0x00 0x00 0x45];
write(serialObj,dataframe1,"uint8");
response = read(serialObj,13,"uint8");
write(serialObj,dataframe2,"uint8");
response = read(serialObj,13,"uint8");
sys = [];
function sys=mdlTerminate(t,x,u,serialObj)
delete(serialObj);
sys = [];

it worked successfully!

However, I still have a problem: during the simulation, the simulation time progress bar is not real-time. I set simulation time be 10sec, but actual execution time exceed 10sec. Could this be due to the inefficiency of the code? because I want real-time communication.