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Hewlett‘Packard’s HP 495IC set up ready for action, with its main menu screen visible. All functions are easily selected using menus and the six

function keys just below the screen.

Ievels at the sendiiig end, sent ovcr at the physical lcvel, and then must work its way back up up to level 7 at the re-ceiving end — all automatically!

Even this is oversimplifying things quite a bit, because it will quite often' move between the lower levels again during the joumey, as it passcs through modems, multiplexers, packet assembler/disasscmblers, and so on.

Hopefully you can now see where the protocol analyśer comes into the pic-ture, to let you see what is going on at each of the levels of protocol, and help track down the causes of any problems.

The Hewlett-Packard HP 4951C is a good example of one of the most popular protocol analysers in current use. As you see from the photographs, it looks very much like a portable Computer, with its smali CRT display screen, flip-down keyboard and built in 3-1/2” mi-crofloppy drive. That’s not surprising, I guess, because it really is just a dedi-cated Computer — like so many of today’s test instruments.

In fact some of the fancięr protocol analysers are essentially very powerful computers, with the equivalent horse-power of about three or four IBM PC-ATs.

The main distinguishing feature of the HP 4951C on the outside is the inter-face pod, which is used to couple the analyśer into the Communications cir-cuits you want to examine. There are actually a number of different pods available, each designed to suit different kinds of physical level protocols. Each pod is built into an alternative clip-on lid, which fits over the folded-up keyboard for transport. The pod shown is the one for RS-232C/V.24 protocol, which is the one most often needed by most users. There are others for RS-449/422A/423A and V.35 protocols.

The RS-232C pod shown also includes a ‘‘breakout box”, to let you play around with the various physical con-nections in the RS-232C Circuit. There are also two sets of LEDs, to indicate basie linę conditiońs.

In use, the HP 4951C can actually pcrfOrm three different kinds of test:

(1) It can be used like a scope or a logie analyśer, to monitor the data Communications taking place between the devices at each end of a Circuit; or

(2)    It can be used to simulate the device normally connected to either end, send-ing various kinds of test data messages for you and then showing you the re-sponse from the other end; or

(3)    It can perform a series of spccial bit-error-rate or BERT tests on the data Circuit concemed, using test data signals designed to allow it to measure the

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Fig.1 (top): The seveh protbeol levels forming the OSI model for data communication. Fig.2 (above): Data moves down to the physical level for transmission, then back up again.

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A close up of the HP 495IC’s interface pod, which includes a "breakout box". The DB-25 connectors at top and bottom right are used to connect to the Circuit under test.

113

ELECTRONICS Australia. September 1987