For terminal connections on VAX VMS and VM/CMS, assuming that there are no special instructions in the Workstation Description Table for the device, any number in the range from 1 to 90 may be used for the conid in calls to GOPWK (but see note below). On VAX VMS, GKSGRAL uses an automatically-generated logical name to connect to the terminal, but it is possible to override this with the command:
DEFINE GKS_DEVICE_n TTxx:where
n is the conid to be used and 'TTxx'
(or 'TXxx', 'LTxx', 'RTxx', 'NVxx', etc.
depending on the type of controller)
is the name of the terminal.
Note that this will only allow use of a terminal other than the one on which
the user is logged in if the user has the requisite privileges,
or if the second terminal is not protected.
Moreover, if the second terminal is not hard-wired, for example, if it is
connected via a Local Area Network terminal server, then one must take care
at the time of use to find out the correct name of the controller and port
to which the physical terminal is connected.
If, for debugging or other purposes, one does not want to get any graphical output on the terminal (and no graphical input is requested), then one can connect to the NULL device with the command:
DEFINE GKS_DEVICE_n NL:where n is the conid to be used.
On VMS, it is possible to capture graphics output command sequences from the
HP Plotter, PostScript (see section on Page 
for information
on Encapsulated PostScript) and Tektronix 4014 drivers on a file.
To do this, open a file with unit number = n and define the
connection identifier to be conid = (100 + n),
where n is a small integer greater than 1.
If there is sufficient interest,
this feature could be extended to other devices.
At CERN, the GKSGRAL and GKSGRAL-3D packages reserve FORTRAN
Logical Unit Numbers (and hence conids) 91-93 for internal use.
Apart from these three numbers the actual range of permissible values
for connection identifiers is in fact 1-100.