plotters again

Tony Duell ard at p850ug1.demon.co.uk
Sat Feb 24 18:05:54 CST 2007


> > If the 9872 is like that, it should be very easily possible to work out
> > what the 9815 interface is doing, since presumanbly it's also mostly
> 
> I had no qualms or questions about what it is doing, it appears quite
> straightforward.
> 
> > handled in hardware, possbily even much the same hardware. From what
> > you've said, it must be more than just a listen-only (unaddressable) HPIB
> > interface, since the plotter can send data back to the 9815
> 
> Try re-reading my previous replies. You're making it more complex than it is.

It would do me no good to re-read the repliece. I know what you've said, 
but _I_ have a problem with it. What you're describing is, I beleive, 
technically impossible.

I believe you are claiming a bidirectional data transfer over an HPIB 
(IEEE-488) line with no addressing of devices. 

Now, from all I've read (indluding the original IEEE488-1975 standard) 
and all I've ever worked with, addressing is what determines the 
direction of data transfer. The device addresed as a talker outputs data, 
the device(s) addressed as listener(s) input said data. I _has_ to be 
that way, becvause there is no particular device that has to be invovled 
in every transfer. It may be convnetional to always have the controller 
(computer) taking part in every data transfer, but it certainly is not 
required. The controller could address one device (say a voltmeter) as a 
talker, another device (a printer) as a listent and then 'sit back' while 
the readings from the voltmeter were printed out.

So there io no 'read/write' line or equivlaent on the HPIB interface.

> The 9815 interface connection consists merely of:

I've looked at hpmuseum.net. Alas the 98130 (olotter interface) manual is 
not there, so I can't see shcemaitcs of that unit. And the schematics of 
the 9872 are quite hard to read -- I hate reading scanned schematics on a 
screen anyway (and no, this is not because I am trying to read them on my 
text-only display :-)). But anyway...

>   - the 8 HPIB data lines (bidirectional)

Actually, I thought it was only 7 data lines (DIO8 is not wired to the 
9815 connector). That makes sense, HPGL is sent in ASCII, which is a 7 
bit code.

>   - the 3 HPIB handshake lines (bidirectional)
>   - a reset/device-clear line
>   - a static 9815-mode assertion line
>   - common

Looking at the scheamtics, the 9815 interfaces uses the same hardware as 
the HPIB interface (the appropriate pins on the 2 connectors are wired in 
parallel). So the handshake must be the nroaml HPIB one, since all that 
is handled in hardware anyway.

My guess, and it's only a guess, is that it works something like this : 

At power-up, if the 9815-mode line is asseeted, the plotter goes into 
listen mode (the HPIB talk/listen modes can be controlled from the 
plotter's microcontroller, this is also used for self-tests on the HPIB 
interface, as it is on the 7245...). The 9815 talks and sends it data.

If the poltter gets a command that should output data (say a digitise 
command), it goes into talk mode automatically and sends said data. The 
9815 side of things knows it's just sent such a command and goes into 
listen mode to recieve said data.

-tony



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