Thanks for the response. Yes, you said correctly.
I think, having seen the relevant documents,
null modem cable seems to be required in this
case. Null modem or cross cable is required
to establish a connection in between two DTE
devices and both computer and terminal are DTE device.
The special jack you mentioned should be DEC's mmj.
Apart from two mmjs, VT520 has two DB25
(one male and one female).
I'm looking for cat5 because it's cheaper.