plotters again

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. 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... 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. 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