232 Serial comm

Thread Starter


Joined Mar 1, 2008

Need some advice...
I am doing asm programming and trying to setup a null modem connection between 2 pc using rs232 serial ports to transfer data from one pc to the other.

i' ve test my hardware(self fix, null modem) using windows hyer terminal and it works well.
next i test my asm program for both pc, using kernelpro advanced virtual com port. i 've test both program, program 1 as com1, program 2 as com2 and it works with a virtual null moderm,com port.

now when i try to run the programs on both pc, it doesnt works.
i've tested it a few times, 1st time it works, the rest of the time it doesn't works. the only difference between the time it works and doesnt works is the type of PC i am using(P4 to P4, P3 to P3).
but is this the cause?

the data i am transferring from pc1 to pc2 are define in my prg1 at pc1.

hope i can get a reply real quick.
thanks in advance!


Joined Aug 15, 2007
Some PCs are picky about the "other" signals on the 232 line. DTR, RTS, etc. This is often seen with a DIY cable with only the GND, RxD, & TxD lines. Google around for which signals need to be connected. Oh, and do you have a "non-virtual" (i.e., real) null modem in use? You'll need it for real hardware. Finally, go to Radio Shack (or its equivalent) and get an RS232 analyzer. I got mine for about $25. You may need 9-to-25 pin adapters.


Thread Starter


Joined Mar 1, 2008
my DIY cable have all the DTR,DSR,CTS,RTS,TX,RX,GND correctly connected. Real null modem? in between my two pc, there is only my DIY cable connecting both.


Joined Nov 25, 2008
"i've tested it a few times, 1st time it works, the rest of the time it doesn't works."

If that statement is a fact then a null modem cable will not solve your problem. If null modem wiring is required and you don't have null modem wiring presently then it can never have worked, not even once. If it worked once and you change from normal to null modem wiring then it will never work again.

If it worked once then you need to make sure your use of the flow control signals, software settings, etc, on both ends of the channel are correct.


Thread Starter


Joined Mar 1, 2008
i pretty sure my settings are correctly, the only difference is the type of PC i am using...
i tested with another 2 pc, one p4 2ghz, one p3 800mhz. firstly i set up both pc with hyperterminal and my null modem, no problem on both side when transmitting/receiving. next i run my asm program, p4 as server, p3 as client. client is suppose to send 3 bytes to server and server will return the 3 bytes.
i experience this:

1) i have to execute my client program 3 times, server program 1 than i can complete my intended transmission.

or, with the same settings:

2) my client fails to display the receive bytes on screen until i shut off the server connection. client is suppose to display the bytes once it receive it back from server with the connection still on.

what i suspect is lagging.