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Serial connections


Hi There;

Does anyone know how to prove a serial interface and serial cable are operational in a lab? Can I use a pc's com port to connect to a serial interface on a router so I can prove that the serial interface and cable on the router will be operational when it gets to site? Is there some sort of serial simulation software that can be used?


3 Replies 3

Goutam Sanyal


Assuming that you are using V.35 and G703 modem in your lab. With that you can connect another router back-to-back. Where it will work and you can check the required.

I don't have any idea about to connect the PC's communication port with the router serial interface, but can guess that would not work because that com port will not provide any clocking for router serial interface.




I am using a smart serial to DB25 DTE male connector. This is in turn connected to a null modem to provide cross over capability with the pc's db9 port which I assume is DTE as well. Using the command 'show int serial' - the last line shows 'DSR down,' whereas dcd, dtr, rts and cts are all 'up.' Can clocking not be provided via the router or does that then make the router a 'dce' device? What settings does a modem have in it that the pc does not?



When on a Serial interface something else might be the problem. Serial interfaces operate in pairs and there usually is a CSU/DSU operating between them along the media. So you need to investigate the signals at the bottom of a show interface.

·DCD = Data Carrier Detect (Status of data carrier detect signal: V.35)

·DSR = Data Set Ready (Status of data set ready signal: V.35)

·DTR = Data Terminal Ready (Status of data terminal ready signal: V.35)

·RTS = Request to Send (Status of request to send signal)

·CTS = Clear to Send (Status of clear to send signal)

When you connect two routers via a crossover cable one router will be the DTE (Data Terminal Equipment) and the other router will be the DCE (Data Circuit-Terminating Equipment).You need to configure the clocking on the DCE. This is accomplished by entering this command router#(config-if)clockrate.

When the routers are connected via a WAN link both routers are the DTE.With CSU/DSU's acting as the DCE providing the clocking.

The DTE is responsible for DTR and RTS. The DCE is responsible for DCD, DSR and CTS.

If you have any more query please back with the output of the serial interface.



Please rate if I am informative!!!

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