Connecting Systems Without An X.25 Network

Overview

In most TSG X.25 installations, the PC is connected to an X.25 packet-switching network by a modem. When one system places a call to another, the Call Request packet crosses the network before being presented as an Incoming Call to the other system.

This topic describes the changes to the normal configuration to be used when connecting two systems directly (that is, without connecting each to an X.25 network). Such setups are useful for testing purposes when no network is available and when the systems are close together. The changes described in this topic are also useful for connecting two links of a card in the same system in loop-back fashion.

Physical Connections

The connectors on each of the X.25 cards are electrically EIA 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 a DCE, so the normal (card to modem) connection is no problem.

However, you cannot just plug the cable from one X.25 card into another card. Both connectors are male. Also, the data transmitted from one system would collide with the data transmitted from the other system 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 TSG 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 TSG 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 system 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 system to itself, we usually connect link 0 to the "I" end of the I-E cable.

The I-E cable can be purchased from The Software Group or you can make your own from the diagram we provide in an appendix of the Installer's Guide. You can also connect the two systems 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).

Software Configuration Changes

On the INTERNAL system, you need to change the link configuration to include:

        speed   9600
        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, do not include the speed line and do not 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, check the maximum speed supported by your TSG cards.

If you are using NetcomRouter to deliver TCP/IP data over the X.25 link, you will also need to set the dna field in the link configuration. See Local X.25 Address for details.

The changes you have made can be activate by running x25net restart

Finally, check your configuration by running tsgstat and selecting the Link Configuration (non-Packet) option. If the first line does not say "The Link is UP", you have a problem, and need to refer to the X.25 Troubleshooting section of the Administrator's Guide.

Further Testing

That is all the changes required to connect two links together. All the commands that you are accustomed to using will work without any noticeable difference. For example, the gentalk utility is useful for stress 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 sink utility receives and checks the data.

Revision 6.1.0 (February 2003)