Benson 1645 plotter experience?

Anyone ever used a Benson 1645-R plotter? We dragged one out of store at the museum the other day. It does its test plot fine, and we look to have got a terminal talking to it as far as handshaking and sending characters via the SA interface. It's not responding to HPGL commands at all though - in fact it's not responding to anything sent down the serial line, other than the handshaking being ok. We're using the 'SA' interface which has one RS232 port and one unknown 9 pin port. We also have an 'SW' interface module, which has two ports on it - one male and one female. According to the (nearly useless) documentation we have, one is for a terminal and one for a host computer - but it doesn't explain the point of that (i.e. whether we should be trying to drive the thing from the terminal port or the host port). Nor does it explain what the difference between 'modem' and 'hardware' connections are (surely the plotter knows nothing of what's actually connected to its interfaces and doesn't care), what the difference is between local and remote mode (this is different between manual and auto mode), what the difference is between normal and emulate mode (emulate what? Selection of Benson or one of various HP plotter types is a seperate menu option, so I don't think it's to do with Benson/HPGL protocol emulation) Nor do the docs say what the difference is between the flow control options on the SW interface (partly why we gave up and started using the SA one) - there's a choice of CTS1, CTS2, then XON 1-4. How those map to the real world we don't know. Grumble grumble. Anyone know how to kick the thing into HPGL emulation mode so we can actually talk to it? cheers Jules