Testing the communications between interfaces of data equipment is a common task when a new premises data communications network is being installed. Such testing usually involves both monitoring and emulation. With monitoring, the network installer wants to view the operation of at least the lower protocol layers of the governing network architecture. That is, the installer wants to see if the proper signals and messages are being passed on the physical, link and network layers. With emulation, the network installer generally wants to test the reliability of specific interface-to-interface links. This is accomplished by generating test messages which 'look as if they are coming from a data device.' These test messages are then transmitted out on an interface-to-interface link. Bit error rate on the link can then be easily measured since the content of the test message is known at the receiving end.

There are many types of test equipment that can carry out monitoring and emulation appropriate for network installation in the premises environment. However, cost and convenience are often an issue. Appropriate available equipment is often very 'pricey', costing thousands if not tens of thousands of dollars. Such high priced equipment may also present 'size-space' problems. It may be appropriate for the laboratory bench, but it may also be inconvenient for a network installer to take out to the field. At the other end, there is also available equipment that is extremely inexpensive but doesn't do much. Examples are simple software-based monitors presented as floppy disks that can be 'popped' into PCs.

The Model 904 PC Notebook Comscope™ Protocol Analyzer presents a very attractive solution to this need for an appropriate test unit. It carries out the appropriate monitoring and emulation functions and comes at a reasonable price - under $500. In order to carry out the appropriate processing for monitoring and emulation it must work with a PC. It is particularly suited to work with Notebook computers that are often taken employed by field installation personnel. In fact the unit can run in the field for up to three hours on a standard 9-Volt battery.

To begin with, it is a small portable unit. It has a built-in RS-232 interface, and an adapter allows it to be used with a V.35/RS-422 interface. It monitors and emulates the most popular protocols: X.25, HDLC, Async and Bisync. It has a built-in BERT (Bit Error Rate Tester). Front panel switches allow the selection of monitoring or emulation of either DTE or DCE. Eight status LEDs display TD, RD, RTS, CTS, DSR, DCD, DTR and RI.

In short, the Model 904 is a high performance data line monitor, BERT and emulator. It is small in size and works with almost all PCs.

The Model 904 uses the PCs com port as the communications link between the resources of the PC and the microprocessor within it.

The unit can monitor data lines on a full duplex basis at data transmission speeds up to 38.4 KBPS on a full duplex basis. It can also emulate data lines at data transmission speeds up to 38.4 KBPS.

The Model 904 is pictured above being attached to a PC.

The illustration above shows how the Model 904 can be employed to monitor a the traffic on a link where data equipment is using the RS-232 interface.