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overail quality of the linc.

As a data monitor, the analyser is simply connccted across the norma! Communications linę. It can then be used to examine the Communications taking place, at any of the appropriate levels.

How does it know all of the protocols operating, in order to lock onto the data and show you what is happening? Weil, if you already know the protocols (often this is the case), you can simply feed the parameters into its memory — rather like selecting the right ranges on a scope.

With the HP 4951C this is very easy, because virtua!ly all of the instrument^ operation is controlled by software menus. So all you have to do is sclect “setup” modę from the main function menu, and then select the various com-munication parameters from the menus.

If you don^łt actually know the proto-cols that are operating, the analyser will actually work them out for itself, using its built-in intelligence. This is known as Auto Configure, and again you simply select it from the main menu . .

As well as letting you see what is going on in real time, the monitor modę also Stores all of the Communications data it finds in an internal buffer memory. So you can recall it after the eveht, and examine it in detail — rather like a storage scope, except that you can “replay” the recording in real time, virtu-ałly recreating a whole scquence of events!

If you wlsh, you can tell the analyser to look for special events, such as particular data bytes, destination ad-dresses, and so on. It can be told to count clock pulses after a certain linę changes level, or look for a combination of conditions on the handshaking-lines, or whatever. There*s great flexibility — and all easily controlled using simpie menu selections.

As a Simulator, the HP 4951C can be set up to act as almost any kind of data Communications devicc, of cither main kind: a Data Communications Equip-ment (DCE), such as a modem, or a Data Terminal Equipment (DTE), such as a terminal or a Computer.

• The great feature here is that you ćan program it to perform in any number of ways, sending out handshaking and data signals in virtually any desired fashion. You can specify the data messages to be sent, how and w hen it is to be sent out, what tó do With incoming data, and so on. And all this is done using simpie on-screen menus, which guide you all the way. It*s surprisingly easy.

Then when you’vc set up a program to perform the simulation you want, you simply tell it lo run, and it springs into action. As the program is running, the screen shows you exactly what is happening — very much like a logie analyser. You see the signal levels on the handshaking lines, plus the data codes on the main signal lines (see

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•

Above: Setting up a program for simulation is quite easy — you are guided all the way by menus which redefine the six function keys. Below: When a simulation program is running, the screen shows you handshaking signal levels as well as the data on the main signal lines.

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screen photos). And of course, every-thing is also being stored away in the memory, to allow you to analyse it at your leisure later.

For BERT testing, the HP 4951C can be set up for either one-way tests with an analyser at each end, or for up-and-back testing using a single analyser at one end. In the latter case, the Communications Circuit must be looped-back at the far end.

Incidentally the HP 4951C has a non-volatile memory, so its setup data, simulation programs and test data are all retaincd in memory when the power is tumed off. This makes it very con-venient for field testing.

So that I could find out for myself how easy the HP 4951C is to drive, EA was loaned the unit shown in the photos by Tech-Rentals, the cquipment rental company. Although I*ve never actually driven a protocol analyser before, I found it very easy to usc — thanks to the logical menu drivcn operation. After only a couple of hours I found myself setting it up quite confidently for simulating a modem, and sending test messages to my IBM PC running a Communications program.

Actually I did have a bit of trouble getting the IBM to respond — not be* cause there was any problem driving the HP 4951C, but because I couldn’t remember how to configure the IBM comms program!

I found the flexibility of the HP 4951C very impressive. It’s so easy to vary virtually any parameter you fancy — baud ratę, handshaking signal timing, test message content, the data codę used — you name it! It’s all under your control, by keying in a few simpie com-mands.

Even from this short test drive, you get a much clearer idea of just how powerful a tool the protocol analyser is for tracking down problems in data Communications systems. In fact as we go. further and further into the data Communications era, I can see these in-struments becoming virtually essential for any kind of efficient troubleshoot-ing.

If you need one just to help you sort out a few curly problems, the Tech-Rentals people have them for rent by the day. They have offices in each State, where you can get catalogs and further details.

Needless to say, firms like Hewlett-Packard are also able to sell you one, if you have a morę ongoing need. The model HP 4951C currently sells for around S8(KK). Hewlett-Packard also has offices in each State.

ELECTRONICS Australia. September 1987

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