Verilog - "Illegal output or inout port connection for port"

Question

This is my first experience coding in Verilog and also my first StackExchange query! Please excuse me in advance for any etiquette I fail to employ in this post.

I trudged through some similar questions already posted here, but I couldn't make out how to apply the solutions to my own code...

I am unsure as to why I am receiving the above stated error when attempting to run a simulation, and am also unsure how to fix it. Please advise?:)

I've attached my source code below, along with the testbench module and the errors I received when trying to run the simulation.

Any feedback is much appreciated!

---

module test1();
   reg O, P, W;  
   wire LowRate, StandardRate, PeakRate;   
outputs LowRate,StandardRate,PeakRate


   CircuitStructure 
testboi(LowRate,StandardRate,PeakRate,O,P,W);

   initial
   begin


O=0; P=0; W=0;
#10 O=0; P=0; W=0;
#10 O=0; P=0; W=1;
#10 O=0; P=1; W=0;
#10 O=0; P=1; W=1;
#10 O=1; P=0; W=0;
#10 O=1; P=0; W=1;
#10 O=1; P=1; W=0;
#10 O=1; P=1; W=1;

#10
$finish();
end
endmodule


module CircuitStructure(O, P, W, LowRate, 
StandardRate, PeakRate);

   input O, P, W;  
output LowRate, StandardRate, PeakRate;

   not
    UA1(NotP,P),
    UA2(NotO,O),
    UA3(NotW,W);

   nand
    UB1(Nand1,NotP,NotO),
    UB2(Nand2,NotW,P),
    UB3(PeakRate,Nand1,Nand2);

   and
    UC1(StandardRate,P,W);

   buf
    UD1(LowRate,O);
endmodule

---

Simulation Errors:

Loading work.test1
# Loading work.CircuitStructure
# ** Error (suppressible): (vsim-3053) 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v(10): Illegal         
output or inout port connection for port 'LowRate'.
#    Time: 0 ns  Iteration: 0  Instance: /test1/testboi File: 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v
# ** Error (suppressible): (vsim-3053) 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v(10): Illegal 
output or inout port connection for port 'StandardRate'.
#    Time: 0 ns  Iteration: 0  Instance: /test1/testboi File: 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v
# ** Error (suppressible): (vsim-3053) 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v(10): Illegal 
output or inout port connection for port 'PeakRate'.
#    Time: 0 ns  Iteration: 0  Instance: /test1/testboi File: 
C:/Modeltech_pe_edu_10.4a/ECE171_Project1/TestBench - Copy.v
# Error loading design

Answer 1

You have defined your module as:

module CircuitStructure(O, P, W, LowRate, StandardRate, PeakRate);

However, defining your unit test, you use different signals order:

testboi(LowRate,StandardRate,PeakRate,O,P,W);

That's why compiler assumes you want to assing LowRate signal to O input, StandardRate to P input, etc. IEEE Standard 1800-2017 (ch. 23.3.2) define following ways to connect modules instances:

• Positional connections by port order,
• Named port connections using fully explicit connections,
• Named port connections using implicit connections (SystemVerilog),
• Named port connections using a wildcard port name (SystemVerilog).

Using the first one, you need to change your signals order:

testboi(O,P,W,LowRate,StandardRate,PeakRate);

Using the second one, you need to explicitly "tell" compiler which signals are assigned to particular ports:

testboi(.LowRate(LowRate),.StandardRate(StandardRate),.PeakRate(PeakRate),.O(O),.P(P),.W(W));