procedure TReadThread.Execute;
var
ComStat: TComStat;
dwMask, dwError: DWORD;
OverRead: TOverlapped;
Buf: array[0..$FF] of Byte;
dwRead: DWORD;
begin
OverRead.hEvent := CreateEvent(nil, True, False, nil);
if OverRead.hEvent = Null then
raise Exception.Create('Error creating read event');

FreeOnTerminate := True;

while not Terminated do
begin
if not WaitCommEvent(hPort, dwMask, @OverRead) then
begin
if GetLastError = ERROR_IO_PENDING then
WaitForSingleObject(OverRead.hEvent, INFINITE)
else
raise Exception.Create('Error waiting port event');
end;

if not ClearCommError(hPort, dwError, @ComStat) then
raise Exception.Create('Error clearing port');

dwRead := ComStat.cbInQue;

if dwRead > 0 then
begin
if not ReadFile(hPort, Buf, dwRead, dwRead, @OverRead) then
raise Exception.Create('Error reading port');
//Последнее условие выдает ошибку 998: ERROR_NOACCESS,
//как я понимаю, это ошибка доступа при чтении порта.
//
end;
end;
end;

hPort:=CreateFile(ComPort,GENERIC_READ or GENERIC_WRITE, 0, nil,
OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0);

if hCom=INVALID_HANDLE_VALUE then
  raise Exception.Create('Error');

SetupComm(hPort,1600,1600);// назначение этих параметров мне не очень
//понятно

if not GetCommState(hPort, DCB) then begin
raise Exception.Create('Error');
exit;
end;

with DCB do begin
   BaudRate:=BRate;
   ByteSize:=D_Bits;
   Parity:=P_Control;
   StopBits:=S_Bits;
end;

if not SetCommState(hPort,DCB) then
   raise Exception.Create('Error');

if not PurgeComm(hPort, PURGE_TXCLEAR or PURGE_RXCLEAR) then begin
raise Exception.Create('Error');
exit;
end;

if not SetCommMask(hPort, EV_RXCHAR) then
raise Exception.Create('Error setting port mask');

Project RS232COM.exe raised exception class EOSError with message 'System Error. Code: 998.
Неверная попытка доступа к адресу памяти'. Process stopped. Use Step or Run to continue.

new := TReadThread.create(true);
new.freeonterminate := true;
new.priority := tplowest;
new.resume;