저는 MSComm을 사용했어요. Active 컴퍼넌트 추가에 보시면 설치하실 수 있어요

void __fastcall TMainForm::Button1Click(TObject *Sender)
{
         MSComm1->CommPort = StrToInt(ComPort);
         MSComm1->Settings="9600,N,8,1";
         MSComm1->Handshaking = 0;
         MSComm1->RThreshold  = 1;
         MSComm1->RTSEnable   = true;
         MSComm1->PortOpen    = true;
}
void __fastcall TMainForm::MSComm1Comm(TObject *Sender)
{
   AnsiString ReadBuffer;

   ReadBuffer = "";
   if ( MSComm1->InBufferCount < 1) return;

      ReadBuffer = OleVariantToString(MSComm1->Input);
   }

   Memo1->Lines->Add(ReadBuffer);
}
String __fastcall TMainForm::OleVariantToString(OleVariant ovSource)
{
   WideString wsTarget = ovSource;
   return((String)wsTarget);
}
조금은 틀릴지 몰라요 제가 가진 소스에서 갈취한것이라서요...
하지만 문법은 맞을거에요