Interfaces s0/s1 are not coming up on cisco 2500 Router

I

imi6969

Im trying to to set up cisco routers (2500 series) for CCNA studies but due to some reasons the serial interface s0/s1 are not coming up.

Before assigning ip address ,clockrate at DCE interface and no shutdown commands the show interface command says the following:
Serial0 is administratively down, line protocol is down
Hardware is HD64570......

And after ip address,clockrate,no shutdown command ,
show interface reads like
Serial0 is down, line protocol is down........

show controller command does not show the type (DTE or DCE) interface.(as it shows in simulator)
I changed the cables changed the routers (I have four) , reload the routers, even set back every thing to default factory default settings but nothing works.

One thing I noticed that the led were off on all seria interfaces on all four routers but ethernet leds were flickering.
Your professional advice/help would be highly appericiated.

Thanks

(I managed to configure the routers , completely in the simulator but not with real hardware)
 
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