Writing bits to COM port in Windows - equivalent to INPORTB function - c++

Question

I am writing a program to control a relay switch connected thru the serial (RS-232) COM1 port. The device I am using has contains two relay switches. These are either 'open' or 'closed'.
By default, both relays are open. The number 1 relay is closed by setting bit 1 in the modem control register. Number 2 relay is closed by setting bit 0 in the modem control register.
In C, this can be achieved as follows:
x = inportb(0x3FC);
x=x & ~2; //Set bit 1 to zero
x=x | 2; //Set bit 1 to one
x=x & ~1; //Set bit 0 to zero
x=x | 1; //Set bit 0 to one
C++ however, does not make COM port access so easy (or so I am led to believe). I currently have the following code, which successfully opens and closes the COM port so that it can be communicated with in C++ and Windows:
#include <windows.h>
//Initialise Windows module
int WINAPI WinMain (HINSTANCE hThisInstance, HINSTANCE hPrevInstance,
LPSTR lpszArgument, int nFunsterStil)
{
//Define the serial port precedure
HANDLE hSerial;
//Initialise relay
hSerial = CreateFile("COM1",GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
//SOME CODE MUST GO HERE
//Deactivate relay
CloseHandle(hSerial);
return 0;
}
However, I am at a loss as to how to 'set bits'. I'm still very new to C++. Any help would be gratefully appreciated.

Answer 1

You need to understand how your device works. You cannot "set bits" directly in hSerial. Having COM port opened, you can send/receive bytes. Possibly you need to send something to device using WriteFile(hSerial, ...). Maybe you need to read information from device using ReadFile(hSerial, ...). COM port is not port as it was called in DOS, this is not address or register. This is input/output stream. You work with COM port like C program works with STDIN and STDOUT - read and wrire information from/to port.
Once you understand what your device needs (this means, define communication protocol), implement it in the program. Read this article: Serial Communications http://msdn.microsoft.com/en-us/library/ff802693.aspx - everything Win32 programmer needs to know about serial communications.

Answer 2

I am presuming that C example you give is not actually on a Windows box and is from some embedded platform is that correct? As it looks like you have direct access to the COM port hardware.
It is not as such C++ that makes this more challenging it is that Windows will not give you direct access to the hardware as you have on your embedded system. It is in fact not possible to set the level of specific pins on the COM port through the standard Windows API, you would need to write a driver that runs with increased privileges to achieve that. It might be impossible to do what you are trying to without writing a driver, something I wouldn't recommend.
The best solution would be to write the embedded end to accept commands as data sent over the RS-232 rather than look for levels on specific pins. The answer from Alex Farber provides a good link to get you started with serial comms in windows.

Answer 3

Not 100% sure what you are doing here, or what you want. The data on the serial port is, well, serial and you cannot drive relays directly by sending data to the UART tx buffer on 0x3FC. That and, as pointed out by other posters, access to the IO map is privileged since W95/98/ME/otherWintendo.
There may be good news - if you only need to drive two, (high sensitivity/low current), relays, you can do it by utilising the RTS and DTR control lines. The current available from these pins is enough to drive a 12V coil via. a diode, (line shifts from +12 to -12 so you need the diode on an ordinary non-polarised relay plus, if the relay does not already contain one, another diode to clamp any back EMFs during switching).
RTS/DTR can be driven from the serial APIs without any special privilege.
I am doing this now to simulate a flow sensor switch when testing an embedded controller.
Note that the relays cannot be toggled at any great rate - the IO is very slow. I can get about 15Hz on a good day. 4*4GHz cores, 12GB of RAM and my PC can only toggle 1 bit at 15Hz!
Rgds,
Martin