Hello hsnaik

Ring Topology on Profibus is possible but requires a fiber optic ring and fiber optic Profibus devices in the ring that cater for this and have the ability to manage the traffic in the ring.

It was sheer luck that it worked with your copper based ring Network without the Repeater in it. Put simple, every device has a loop through plug and if the device sends a message on the bus, the message will travel out on both ports of the plug and "collide" with itself on the opposite end of the ring. Depending on baud rate and cable length the electrical signals may then cancel each out out there and your pure "solid" copper ring may in fact work (as said though, you simply got lucky). By adding a repeater, you now find that it won't work anymore as the repeater does is best to regenerate the message that it gets on either port and spit it out on the other port, thus potentially keeping the same old message flowing for eternity which means your whole ring breaks down.

Anyhow, what you can do is:

Build a Siemens OLM based fiber optic ring, see the OLM manual for more

Build a PROCENTEC ComBricks based fiber optic ring with the additional 24/7 Network monitoring advantage, see http://www.procentec.com/products/combricks/modules/profibus-multi-mode-fiber-optic-ring  for more on this

Search for Profibus fiber optic ring capable devices from other manufacturers