Straight vs Null Modem Serial cable confusion

Question

I have a cable having a DB 9 male at one end and a female at another end. I will be using this cable to connect BeagleBoard Xm ver C1 to the serial port of my PC. I understand the pins with respect to the following image:

enter image description here

Now this document says that I must have a Straight-through Serial cable....

The BeagleBoard-xM provides a female DB9 port and requires a serial cable to connect the console to your host system. Use a straight-through (not null-modem) cable.

I have checked that as per the above image, pins are shorted like this: 1-1, 2-2,3-3, 4-4, 5-5 and so on.

Does that means my cable is a straight-through cable as required?

Another resource is here that explains the type of cable I must have:

Serial Port-Related

    Why is there no output? Why is it not possible to type into the serial port?
        For original BeagleBoard, purchase the recommended serial adapter. The BeagleBoard-xM utilizes a straight-through DB9 male-to-female serial cable.
        Ensure that you are connecting:
        RX to pin 2
        TX to pin 3, and
        GND to pin 5.

Based on this am I using the correct cable?

score 2 · Answer 1 · answered Nov 06 '13 at 12:41

2

I have checked that as per the above image, pins are shorted like this: 1-1, 2-2,3-3, 4-4, 5-5 and so on.

Does that means my cable is a straight-through cable as required?

Yes. A straight-through cable connects pins as corresponding pairs. Pin 1 to pin 1, pin 2 to pin 2, and so on. "Extender" cables (having opposite sex connectors at the two ends) are usually wired liked that.

A null-modem, or cross-link cable usually has similar connectors at the two ends, and therefore outputs and inputs have to be swapped (crossed) so that an input at one end gets connected to an output at the other end.

answered Nov 06 '13 at 12:41

Laszlo Valko

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in summary can you tell me whether I have the correct cable or not? – gpuguy Nov 06 '13 at 13:25
See the first word in my answer: _Yes._ – Laszlo Valko Nov 06 '13 at 17:05

score 0 · Answer 2 · answered Nov 06 '13 at 12:41

A PC normally uses a male connector that is configured as a DTE (Data Terminal Equipment) which is a reference to traditional terminals being used to control / connect to other devices such as modems which are configured as a DCE (Data Communication Equipment) that normally use a female connector. By convention a PC is still configured as a DTE like a terminal and the peripheral as a DCE.

The following composite image based on The RS232 Standard illustrates the pin functions are different on each end:

enter image description here

So you're correct that a straight through cable should be used in this case. For example pin 2 is received data on the PC (DTE) side while it's transmitted data on the BeagleBoard (DCE) side.

Straight vs Null Modem Serial cable confusion

2 Answers2