HP-85 7470A Plotter

An HP 7470A plotter appeared on gumtree. It’s a serial version rather than the HP-IB version, but i thought i might be able to make it work. The bloke who sold it to me, Bob, was an absolute gentleman – even offering to turn up pen holders on his lathe!

It’s quite a simple little plotter that uses only two pens.

The plotter itself needed a drop of oil on the rail, but otherwise worked fine.

The manual lists the HP-IB version as the one to use with the HP-85, so I had a bit of reading to do to work out if it could work over the serial line. In principle, it could be done by directing the plotter output to the serial line:

PLOTTER IS 10

Many people have reported that this causes the HP-85 to just hang.  There was some suggestion that this was due to the serial version of the plotter sending just a CR rather than a CR/LF at the end of its transmission. (10 is actually set on the serial card.)

https://forum.vcfed.org/index.php?threads/serial-plotting-on-the-hp85.41043

The last post which came very late is key.

The plotter can be programmed to send a CR/LF.  I did the required reading and found that this worked:

PRINTER IS 10
PRINT CHR$(27);”.M;;;13;10:”
PLOTTER is 10

No hang!  I tried some test code and found that I could send HPGL and use the HP 85 plotter commands.  I didn’t have any success with the Graphics Presentation software on disk.  I suspect that it resets the interface.  There may be a workaround for that, but I’m not overly concerned.  I don’t mind writing a bit of BASIC to make the plotter do its thing.

I used 2400 baud, 8 bits, 1 stop bit, no parity as per the serial card.  This is done through DIP switches next to the serial connector.  The serial card and the plotter both had female connectors but the transmit/receive was reversed so I just used a M-M gender changer between them.

The plotter commands are the same as for the screen.  Using PLOTTER IS 10 points them to the plotter instead.

The HPGL codes are in the plotter interfacing manual. 

Comms are more reliable with flow control.  I’ve used xon/xoff a lot in the past – this sets up the machine and the plotter to use xon/xoff.

Xon/xoff at plotter – 7470A Interfacing and programming manual

PRINTER IS 10
PRINT CHR$(27);”.M;;;13;10:”
PRINT CHR$(27);”.I;;17:”
PRINT CHR$(27);”.N;19:”
PLOTTER is 10

Xon/Xoff at computer – 82939A Serial Port Manual

I changed the DIP switch settings on the serial card to suit the datacomms pac, so the settings for the plotter need to be programmed.

CONTROL 10,3 ; 11 ! 2400 baud
CONTROL 10,4;7 ! No parity, 2 stop bits, 8 data bits

The I/O ROM provides a command, TRANSLATE, which converts a program to play nicely with it.  Load the program (eg CALEND in the standard pac, and type TRANSLATE.  It can then be STORED as an external plotter variant. 

For the serial plotter, load and run the instructions above and then load and run the external plot variant.