This article is from the Token Ring FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
Unlike Ethernet stations, token ring stations _cannot_ be directly
attached with a cross-over cable. Because of the process required
for inserting into a ring, a loopback process must complete and
phantom voltage must exist on a wire for a relay to open. A MAU must
be used to directly connect two workstations.
However, some token ring switches allow a station to directly
connect to a _switch_. This Direct Token Ring (DTR) connection is a
non-standard method of connecting a switch and a workstation onto a
single ring. This non-standard DTR connectivity does _not_ allow for
two workstations to be directly connected.