If someone could help me, please, its driving me nuts. So, I have two programs on two separate computers communicating via serial port. it works great, provided that both programs have same serial port settings (e.g. port number, baud rate, parity, ect...) I would like to add a routine which checks if communication between two programs via serial port was established, and data can be sent back and forth. to setup communication, i used MSComm control, and it has some properties which would let you do it (DCHolding, CTSHolding, ect...). As I understand those properties require actual pin corresponding to the property to be in the connector. All serial cables I have gotten a hold of, only had 3 lines in them (send, receive and ground). IS there any way to find out whether the proper communication was established between two programs using only those three lines, before sending and receiving data. I only have control over one program as far as the code, so I cant send and receive a test character. all the checking has to be done prior to sending data. I know that it is not a typical question, however, if anyone has the answer, PLEASE share. Ive been looking for a sloution for 2 days now.

Typically if you use only 3 wires (send, receive, and ground), you would have to jumper RTS/CTS and DTR/DSR flow controls. You can shut off the requirements for flow control in software. But unless the speed is very slow you will probably have buffer overrun and lose data. You can test if you are sending data by running a terminal program (Hyperterm) in the other computer and watch for characters. I would recommend setting the baud rate low (2400 baud) when testing, then gradually increasing speed. To get the highest speed you will have to implement flow control. 3-wire connections are typically used by interactive terminals where the output typically is less than 1 screen. Also remember if you connect computer to computer you need a null-cable.

thanks for responding. the flow control is set to none. and the speed is set to 1200, which is plenty for sending few characters back and forth. as far as the testing connection goes, I would like to avoid using other programs (hyperterm as you mentioned) to test the connection, but rather add a few lines of code which would tell me that its OK to send/receive data BEFORE I do any sending/receiving. the other program (the one I have no control over), was written to use 3 wire serial cable, so I have to comply on my end.

You can't test for a ready condition if you don't use any handshaking (hardware or software). With 3 wires. You can test you receive buffer for received characters, but you cannot test if the other side is ready to receive (unless it continuously sends a signal that it is available or some otherform of software handshaking). Assuming the other program is well-behaved. It must give a standard response sequence. Your program tests and clears the receive buffer. Send a sequence to the other computer, wait for response. Process response and repeat as needed. This is similar to dialing a modem send AT command and wait for expected response. Program logic branches according to received results. Save data, different command, etc. At 1200 baud and using a buffered serial port your computer should be able to process the data stream with no problem. Provided you don't run another application.

tech-fred, thank you for all your replies. its a great pity that it is impossible to do with 3 line serial cable. Unfortunately the communication is initiated on my end, and the idea was to check for ready condition before sending anything. Since we are on the topic, maybe you can answer me the following: suppose I did have more than 3 lines in the cable, A. which line would I need to sufficiently check for ready condition? B. is there a way to check if that particualr line is physically present? C. suppose the line is present, how would I then check for ready condition on both ends? Your comments are much appreciated.

Do you know the response characteristics of the device you are sending to? Does it echo the received characters? What is the input buffer of the device that you are sending to? I once had to use pacing (send characters one by one because strings would overflow the input buffer). Is there a command syntax for the device? Like a modem does it has fixed responses to certain commands? Or is it like a multimeter, where it returns a value of range x to y? Actually I think communications is possible if you can determine the state of the device at any given time. Just don't send too much too fast and wait for it to respond.