Communication between two computers using Null Modem Serial Port Connectin

bhanu001 · 09-17-2011, 07:30 AM

Hi Everyone,

I'm trying to connect two computers using a serial port (using null modem connection) as shown in the figure below.

On one computer (let's call it sender), i'm opening a serial port connection using open() system call as

Code:

int fd = open("/dev/ttyS0", O_RDONLY | O_NOCTTY | N_NDELAY);

and write data into the port using write() system call as

Code:

char *ptr = new char[SIZE];
int no_of_bytes = write(fd, (void *) ptr, SIZE);

And on the other computer (let's call it receiver), i'm opening a serial port using open() syscall and reading the data using read() syscall as

Code:

char *ptr = new ptr[SIZE];
int no_of_bytes = read(fd, (void *)ptr, SIZE);

When the value of SIZE is less than 20 or so, i'm able to read "all" the bytes in one-go. But when the value exceeds 500 or 1000 Bytes (even 100 B, for that matter), I'm unable to receive all the bytes I've sent on the receiver side, even after repetitive read() syscalls !

So please someone shed some light. And thank you for your time.

Bhanu Kiran C

frieza · 09-17-2011, 08:24 AM

define 'connect', that could mean any of a myriad of things.

in what manner are you attempting to communicate between the two machines?

if you are looking for just a command line then you could accomplish the task with agetty on the 'transmitting' end and minicom on the 'receiving' end

if you are looking for full networking then you need pppd on both ends, i forget exactly how to do it but i have done so.
and yes you are correct to use a null modem.

bhanu001 · 09-19-2011, 01:40 AM

Quote:

define 'connect', that could mean any of a myriad of things.

in what manner are you attempting to communicate between the two machines?

i've soldered a wire (null modem connection) to two female serial pins (one on each end) and connected them to both computers (to male serial plugs, one on each end).

Quote:

if you are looking for just a command line then you could accomplish the task with agetty on the 'transmitting' end and minicom on the 'receiving' end

if you are looking for full networking then you need pppd on both ends, i forget exactly how to do it but i have done so.
and yes you are correct to use a null modem.

I have defined a new structure on one computer (size 3000 bytes, which is obtained by reading some information from a file, doing some processing etc) and i would like to transfer the structure to the other computer using a serial port. And after receiving the data, I would type-cast the data into the structure back again ...

Now can you suggest where I'm doing it wrong ???

rodrifra · 09-19-2011, 02:09 AM

Have you read the serial howto?

http://tldp.org/HOWTO/Serial-HOWTO-4.html

I'm asking this because there are a couple of buffers between your app and the port, and it doesen't matter if you define your variable with a size or another if you are not counting with those buffers and how they are fed. I once did a programing communication with a null modem (long time ago, I don't even remember) but I think there was an issue with those buffers to get the correct hole bytes sent.

bhanu001 · 09-20-2011, 01:55 AM

Mr. rodrifra,

Quote:

Have you read the serial howto?

http://tldp.org/HOWTO/Serial-HOWTO-4.html

I've gone through the document and checked the baud rates of both the serial links and checked other details too, everything is fine ... even then i'm losing a lot of data (i mean when is send 100 bytes from one side, i receive only 20 bytes or so on the other).

So what else do I need to trouble shoot ?

Many thanks for your participation.

BhanuKiran C

rodrifra · 09-20-2011, 09:06 AM

In the next link you have programming examples, hope that helps.

http://www.lafn.org/~dave/linux/Seri...ng-HOWTO-B.txt

bhanu001 · 09-23-2011, 12:48 AM

Hi "rodrifra"

Quote:

In the next link you have programming examples, hope that helps.

http://www.lafn.org/~dave/linux/Seri...ng-HOWTO-B.txt

I've followed these tips too, but still I'm losing data. When I send 2000 or more bytes, I receive only 1000 odd bytes.

Please can you suggest some more tips.

Thank you,
BhanuKiran

dilbert_uk · 09-23-2011, 06:20 AM

I am not sure what "SIZE" is. If that gives a wrong number because it's not portable code or what ever else then maybe you could loose characters. Try to use "strlen()" instead or a constant like buffer size.

Then, if you use the same file descriptor for read and write, did you use the select() function from those how-to examples, too?