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Old 04-17-2008, 12:04 AM   #1
Registered: Jun 2006
Distribution: Kubuntu Hardy
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Using USB<->Serial and serial gender changer to transmit data between computers

I would like to be able to send a stream of data from my non-USB laptop to my USB desktop. I bought a USB<->Serial converter and a serial gender changer to try to do this. When I plug in the USB end to my HP Pavilion 503n running CentOS 5, my lsusb says this:

Bus 003 Device 004: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
As well, a /dev/ttyUSB0 appears. The other end is connected via gender changer to a female serial port on a Compaq LTE 5100 running Debian Etch. The problem is that when I do this:

Serial end: cat /dev/ttyS0
USB end: echo hi > /dev/ttyUSB0

The echo on the USB end completes, but nothing appears on the serial end. Sending data the other direction doesn't remedy the problem either. Moreover, I tested this on my main machine, and it had the same result. Fortunately, no system lockups happened during any of the tests.

I'm guessing the problem is that the gender changer doesn't rearrange the pins so that it can communicate this way (on both sides, I see 54321/9876). My question is: am I missing something else? Are my parts defective?
Old 04-17-2008, 12:27 AM   #2
Senior Member
Registered: Jul 2003
Location: Indiana
Distribution: Suse 6.0+, Mandrake 5.0-10.0, Redhat 6.0-9.0, Gentoo 1.2+, Gnoppix, Knoppix, Sabayon, Ubuntu 5.04+
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You probably need to use an actual serial to serial transfer cable as the pins don't neccessarily stay in the same order for direct transfers. It still might not work as the usb to serial was made for people to use things like modems, or sync old pda devices and such to modern computers. It may not be wired the same as an actual port.
Old 04-17-2008, 12:43 AM   #3
Registered: Mar 2005
Location: england
Distribution: slackware, win2k
Posts: 364

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Try getting, or finding out how to make a null modem cable. I'm not sure if it will work in conjunction with a USB-serial adaptor, don't see why it can't tho.

A null modem cable is basicly another name (or is it the proper name) for a direct computer to computer serial link cable. It has the pins wired in the correct sequence to allow the transmit signal on one end to be connected to the receive signal on the other end, etc.

For example i use one of these and minicom to view a serial console on one machine at another machine.

Old 04-17-2008, 01:20 AM   #4
Registered: Jul 2003
Location: Pennsylvainia
Distribution: Slackware / Debian / *Ubuntu / Opensuse / Solaris uname: Brian Cooney
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What are you trying to accomplish? serial login, or data transfer?

If you want to log into the box via serial, you need a null modem adapter

If you want to transfer data, just get a crossover cable and connect via ethernet
Old 04-17-2008, 06:15 AM   #5
Registered: Jul 2003
Posts: 363

Rep: Reputation: 50
April 16, 2008

Hi Joey

My two cents worth is this.

To achieve a PC to PC link over an RS232 serial port you need to use a null modem cable.

Simply put a PC's serial ports are configured as a Data Terminal Equipment (DTE) device.

To connect two DTE devices together you need a NULL modem.

You cannot simply connect two DTE ports together since both DTEs will transmit, receive, and handshake on the same pins.

Hence you use a null modem adapter or null modem cable to connect two DTEs.

What this does is match transmit on one DTE to receive on the other DTE and vice versa; it also connects the handshaking signals in a configuration to allow data transfer.

You can find a configuration diagram on the internet for a basic null modem cable configuration and make one yourself; or purchase a pre-made null modem cable; or use a straight (pass-through; 1:1) serial cable and connect a null modem adapter in line
with the cable.

Using the USB to RS-232(serial) adapter should work.

However, I have one reservation.

I have seen some USB to RS232(serial) adapter cables that are supplied with driver software for use under MS Windows.

Therefore there may be some issues with how the adapter operates.

Wishing you the best of luck.


Last edited by cgtueno; 04-17-2008 at 06:18 AM. Reason: formatting


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