Photon cannons require a link to the psionic matrix generated by pylons. Although the protoss revel in the glory of personal combat, they also recognize the need for automated defenses. As such, the cannon fires an encapsulated antimatter sphere similar to that generated by the dragoon, which can target air and ground targets. Inexpensive and easy to deploy, photon cannons are often installed at the entrances to protoss encampments, or overlooking psi-powered checkpoints. Photon cannons aren't rapid-fire weapons, so they're not as effective against massed foes. A marine rifle company or a pack of zerglings can easily work its way through a protoss station, picking off cannons one by one.
The photon cannon initially structuraly resembled its predecessor, retaining its 'telescope'. It was replaced by the phase cannon early in the development of StarCraft II. It has since returned in name and ability, although uses the phase cannon's model.