Connecting Two Machines Without An X.25 Network

Overview
In most installations of Netcom, the PC will be connected to an X.25 packet-switching network via a modem. When one machine places a call to another, the Call Request packet crosses the network before being presented as an Incoming Call to the other machine.

This application note describes the changes to the normal Netcom configuration to be used for connecting two machines directly (ie: without connecting each to an X.25 network). Such setups are useful for testing purposes when no network is available and when the machines are close together. The changes described in this note are also useful for connecting two links of a Netcom card(s) in the same machine in loop-back fashion.

Physical Connections
The connectors on each of the Netcom cards are electrically EIA RS-232-C DTE (data terminal equipment). You must always make electrical connections between a DTE and a DCE (data communication equipment). A modem is one example of an RS-232-C DCE, so the normal (Netcom to modem) connection is no problem.

However, you can't just plug the cable from one Netcom card into another Netcom card. For starters, both connectors are male. Also, the data transmitted from one machine would collide with the data transmitted from the other machine rather than going into its receiver.

The solution is to use a synchronous null-modem cable also known as an I-E (spoken as "I to E") cable. When the "I" side of this cable is plugged into a Netcom card, it makes the card look like a DCE. That is, the free "E" side has the pinout of a DCE and is a female connector. You can now plug a cable from another Netcom card into this cable.

The names "I" and "E" on the null-modem cable stand for "INTERNAL" and "EXTERNAL". Whenever you connect two synchronous devices, one (and only one) provides the clock (is internally clocked) and the other receives it (is externally clocked). Our convention is to have the DCE provide the clock (just like a network modem). So, make note of which machine is connected to the "I" side of the cable and which is closest to the "E" side, designating them as Internal and External respectively. In connecting a machine to itself, we usually connect link 0 to the "I" end of the I-E cable.

The I-E cable referred to above can be purchased from The Software Group or you can make your own from the diagram we provide in the Hardware Release Notes of the Netcom User's Guide. You can also connect the two machines using two short-haul modems or active null-modems (the I-E cable is a passive null-modem, requiring a clock source on pin 24 of the I end).

Hardware Configuration Changes
You have to change the hardware configuration of the Netcom board in the INTERNAL machine to provide the clocks. On some Netcom hardware platforms, a jumper must be moved to do this. Check the Hardware Release Notes of the card you are using to find out how to change from external clocking (our factory default) to internal clocking (which you need for this configuration).

Software Configuration Changes
On the INTERNAL machine, you need to change the link configuration. Edit /usr/lib/x25/utils/fld.opts About a page down, you will see a line reading -f0 followed shortly by -0 link.telenet (or link.van84 for newer releases) and -1 link.inactive. This says that link.telenet describes the link configuration for link 0 and link.inactive describes link 1. Change link.telenet in the first line to link.int.dce. Copy link.telenet to link.int.dce and add two lines to the new file (anywhere):           speed 96 (or 9600 for newer versions)           dte NO

The first line says the link is internally clocked at 9600 baud (the default, speed 0, means externally clocked). The second line says that the link should act as DCE. This is the "logical" DCE/DTE configuration (affecting layers 2 and 3 of X.25 — not the physical layer that the I-E cable takes care of).

If you are using short-haul modems, don't include the speed line and don't change the hardware configuration, as the modems provide clock signals to both of the Netcom cards. One side of the connection will still have to be configured as X.25 DCE (with dte NO described above).

If you wish to use a speed other than 9600 baud, refer to your Hardware Release Notes to check the maximum speed supported by your Netcom cards.

The changes you have made can be activated by running fld which is found in the same directory. This will also run each time you reboot the machine and should now indicate that it is using the configuration file link.int.dce. You should see the message "Front End Processor #0 Running" on the machine's console.

Finally, check your configuration by running noc and selecting item #4 in the statistics menu. If the first line doesn't say "The Link is UP", you've got a problem, and need to refer to the Netcom Troubleshooting section of the Administrator's Guide.

Further Testing
That's all of the changes required to connect two Netcom links together. All of the commands that you are used to using will work without any noticeable difference. In particular, take note of the programs xpad and talkt which are suitable for remote login from one machine to another.

Also, the utility gentalk is useful for high pressure testing of the connection. Its purpose is to push data across the X.25 connection as fast as possible. At the other end of the line, the utility sink receives and checks the data.