Null modem cable

[cobracon]

What is the difference between a Modem cable and a null modem cable?

 

[Chuck(G)]

RS232 basics, first. There are basically two types of equipment in the RS232 world: Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). A modem is DCE (pretty obvious) and a terminal (or your PC) is DTE. DTE and DCE in addition to data lines also have handshaking lines (e.g. RTS (Request to Send), CTS (Clear to Send)...).

Ideally, if you connect all pins on a connector "straight through" from a DCE to a DTE, everything works the way it should. That would be one form of a modem cable.

However, if you want to connect to DTEs (e.g. two PCs) together, you have a gender problem. So a special cable that cross-connects various pins on one end or the other, makes a DTE look like a DCE, so you can connect two PCs together (or two terminals of any sort).

Wikipedia has more detail

 

[Ole Juul]

What Chuck said.

I find it helps to visualize that a transmitter goes to a receiver and vice versa. Therefore you have to cross two wires to make a null modem cable. Think of putting two telephone handsets together.

 

[Chuck(G)]

I think it's interesting that almost everyone back then used female DB25 connectors whether or not they were DTE or DCE. Strictly speaking, that violates the standard. DTE is supposed to use male DB25; DCE, female.

The IBM 5150 was the first system I actually saw that used the male DB25 connector on the async comm port. To me, that implied that the PC was not intended to drive a terminal, yet there on the card were the jumpers to set it up for EIA current loop host (i.e. it supplied power for a 20 ma. loop). In a practical way, however, it made sense.

 

[g4ugm]

I think it helps to visualize the set-up for which RS232 (or the the similar V24 Europian stanard came from, So we would have had:-

Terminal <== RS232 ==>Modem <=== telephone line ===> Modem <===RS232===>Computer

Now we often run the system without handshaking, but originally the MODEM was the master, so when the two modems had established the link over the telephone line they controlled the flow of data. The modem generates three signals, "Data Set Ready" , "Ring Indicator", and "Carrier Detect" which are used to inicate the state of the connection.

Another pair of lines are used to control fow, the computer generates "Request to send" (RTS) and if the modem is ready to accept data it acknowledes via "Clear to Send" (CTS).

It is however common to want to omit the Modem. When we do this we need to fool the two DTE's that there is a modem there. We use a "Null Modem Cable" for this purpose. Some times we can set the program to ignore the handshaking lines, and in this case we can simple cross the data lines over. On 25-pin plugs the cable goes 2-3, 3-2 & 7-7. If we need to generate the handshakes we need a more complex cable. If memory servers me correctly the ones we used when I worked in a place with real terminals went 2-3, 3-2, 7-7, 4 - (6,8 & 20) and (6,8 & 20)->4.

The wiki page here :-

http://en.wikipedia.org/wiki/Serial_port

has the 9-pin equivalents...

 

[Chuck(G)]

Or just get yourself a serial Laplink cable. Has both 9 and 25-pin connectors on each end.

Cheap and it saves the bother of building your own.

 

[cobracon]

Now this looks like what I need. Thanks guys!

Edit

All I can find is 25pin female to 9 pin female. Anyone sell a 25 pin male to 9pin female laplink cable?

Or would this work with my modem cable: http://www.radioshack.com/product/index.jsp?productId=3964616

 

[MikeS]

Now this looks like what I need. Thanks guys!

Edit

All I can find is 25pin female to 9 pin female. Anyone sell a 25 pin male to 9pin female laplink cable?

Maybe a null-modem, but it wouldn't be a LapLink cable. Maybe you should tell us what you actually want to do? What do you need the 25 pin male for?

Or would this work with my modem cable: http://www.radioshack.com/product/index.jsp?productId=3964616

How can we answer that when we don't know what your modem cable looks like or what you want to plug it into?

 

[cobracon]

I am hooking a Sun server to a pc so that I can use the pc as a terminal. The PC has a 9 pin male and the Sun has a 25 pin female connection. What I have now is a 25 to 9 pin modem cable. I just checked this site : http://www.obsolyte.com/sunFAQ/serial/ and it says dont use a Laplink cable. Thats what I'm doing with it. So I guess I need to know if that radio shack adapter will work with this set up. BTW the servers are as follows: SparcStation 20, Sparc Server 5 and my just acquired Sun Fire 280r.

Robert

 

[MikeS]

Assuming your 'modem cable' is a straight-through 9-pin female to 25-pin male, i.e. the connectors are correct, then yes, that null-modem adapter should work.

 

[cobracon]

Thank you very much! I'll give it a try.

 

[jonnymacuser]

RS232 to RJ-45 fun!

RS232 to RJ-45 fun!

RS232 basics, first. There are basically two types of equipment in the RS232 world: Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). A modem is DCE (pretty obvious) and a terminal (or your PC) is DTE. DTE and DCE in addition to data lines also have handshaking lines (e.g. RTS (Request to Send), CTS (Clear to Send)...).

Ideally, if you connect all pins on a connector "straight through" from a DCE to a DTE, everything works the way it should. That would be one form of a modem cable.

However, if you want to connect to DTEs (e.g. two PCs) together, you have a gender problem. So a special cable that cross-connects various pins on one end or the other, makes a DTE look like a DCE, so you can connect two PCs together (or two terminals of any sort).

Wikipedia has more detail

I'm attempting to hook-up an old televideo ascii terminal to a cisco server; the server has an RJ-45 console port, configured as data terminal equipment (DTE), with straight-thru 100BASE-T pinout:

1............TX+.........1
2............TX-..........2
3............RX+.........3
4...........................4
5...........................5
6............RX-..........6
7...........................7
8...........................8


and the terminal has an RS232 db-25 serial port with pinout:

1 Frame Ground
2 Transmit Data Output
3 Receive Data Input
4 Request Data Input
5 Clear to Send Input
6 Data Set Ready Input
7 Signal Ground
8 Carrier Detect Input
20 Data Terminal Ready Output
25 Current Loop +
>>>>>>Transmit*
13 Current Loop --
12 Current Loop +
>>>>>>Receive*
24 Current Loop --


Now I can get the terminal to display output from the server by connecting the terminal's RS232 db-25 pin #3 "Receive Data Input "
-------> to pin #3 WHITE/Green on a straight-thru RJ-45 cable.

What I can't seem to get working is keyboard input onto the server nor see any characters typed appear on the screen.

Is this solve-able by correct wiring?

 

[modem7]

nor see any characters typed appear on the screen.

By putting yout Televideo terminal into 'half duplex' mode, you will see typed characters appearing on the Televideo screen at the same time as the characters are sent from the terminal. The manual will reveal how to make that change.
FULL DUPLEX: Only display characters received on serial port.
HALF DUPLEX: Display characters typed on terminal keyboard plus display characters received on serial port.

Is this solve-able by correct wiring?

Since you've got comms working one way, the answer is probably yes.

 

[g4ugm]

Johnny you should have really opened a new post for this as it looks like a different question. Firtsly what you say is contradictory. If you get data transfer then the RJ45 port isn't 100BAseT. I suspect the pinouts are as per this:-

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008010ff7a.shtml#hw


Signal RJ-45
RTS 1
None 2
TxD 3
GND 4
GND 5
RxD 6
nc 7
CTS 8

So try connecting Pin 6 on the RJ45 to Pin 2 on the terminal.

 

[jonnymacuser]

... So try connecting Pin 6 on the RJ45 to Pin 2 on the terminal.

solved! thanks G4UGM & MODEM7!!!