Re: [CAD_CAM_EDM_DRO] Hp 7475 Plotter Servo's

Victor,

The null modem adapter basically puts a jumper between RTS and CTS (pins 4 &
5) so that the device sees it's OK to send data when it asks for permission.
In addition, it jumpers pins 6,8 and 20 to provide the required signals for
the two devices to recognize that they are both turned on. it shouldn't
matter wich end of the cable that you put the adapter as long as your serial
cable has all the lines required going straight through. You need pins 2&3
(recieve & transmit), 4&5 (request to send and clear to send), 6 (data set
ready), 7 (ground), 8 (carrier detect) and 20 (data terminal ready).

If the device were "dumb" you could use only 2,3 and 7 and it would work.
However on newer "smart" devices, the computer won't send data if dtr is
not high and the device may not send back if it doesnt see a high on CTS,
etc. etc.

IIRC, I have had problems using the com port under XP - seems Microstupid
decided to keep control of the ports (because they could, and of course,
they know better than we mortals).

Hope this gives some help.

George