When you're dealing with 9-pin serial, the RS-232 standards says that DCE devices should have a female DB-9 connector, and DTE devices should have a male DB-9 connector. That makes them easy to tell apart. But if you really want to tell, find out which pins in your connector correspond to the Data Set Ready (DSR) and Data Terminal Ready (DTR) signal lines. It is the DCE that raises Data Set Ready and the DTE that replies with Data Terminal Ready. So check the voltage on those pins at the device connector, without an RS-232 cable plugged in, and you can tell whether it is a DCE or a DTE :? :? :?
Re: How do I identify if a device is a DTE or a DCE?
14 years 8 months ago #3839
This one caught my attention. I think it has been covered fairly well but I too would like to add a little, as I know how confusing this subject can be.
The only thing I think needs to be said in terms of RS232 is that when connecting two DTE devices you use a Null Modem cable.
The rest of my comments are in terms of Ethernet connections and Router Serial ports.
First a direct answer to the question “How do I tell …” by definition you may need to look it up.
Computers are DTE devices.
Switches, Hubs, Cable Modems, and CSU/DSUs are DCE devices.
Routers are considered DTE devices however they can be used as DCE devices as well, there lies some of the confusion.
Home routers often referred to as gateway routers have mostly switch ports (DCE) and one “Ethernet port” (DTE) usually with a designation to be connected to the cable modem (DCE). Thus you use all straight through cables when connecting your computer to the switch ports of the router and the router to the cable modem.
Routers designed for commercial use will typically have one or two Ethernet ports (DTE). The rest of the ports will vary by model number and many today are made modular. This means you can chose the type of ports you want/need to make “wan connections”
The Ethernet ports (DTE) typically are connected to switches (DCE) or hubs (DCE) then computers (DTE) are connected to the switches. Again using all straight through cables.
There are times you will need to connect computer to computer or switch to switch. You can connect a computer to the Ethernet port on a router however this would be an unusual situation. These all call for crossover cables.
The routers serial port requires a special cable that typically is used to connect between the router and a CSU/DSU in which case you want a DTE cable. The serial end should be labeled as DTE. The other end is usually a V.35 connector with male pins that connects to the CSU/DSU. However you can connect routers together. In this case you need both a DTE and DCE cable. You connect the serial end of each cable to a router and then connect the cables together (the DCE cable will have a female V.35 connector). There are serial “crossover” cables available from
these are special cables most often used in Labs written for the Cisco Academy.
By the way when you connect two routers together the one with the DCE end will need to be configured to provide the Clock. DCE devices determine the clock rate.
I should also note my use of the term Ethernet port is generic and includes FastEthernet and GigabitEthernet. By definition Ethernet is 10Mbs, FastEthernet is 100Mbs, and GigabitEthernet is 1000Mbs. Not all ports or NICs are capable of two or more speeds and speed also has to be matched.