Plasma cannons were a form of directed energy cannon weaponry, the precursor to phase cannons, employed by United Earth and Starfleet vessels during the twenty-second century. Plasma cannons fired a plasma discharge in the form of a beam or burst similar to the plasma bullets fired by hand-held plasma weapons but much larger in size.
In the early to mid twenty-second century, Earth Cargo Service freighters were commonly equipped with low-yield turrets (often referred to as "popguns") designed only to break up asteroids and meteors. Earth Y-class freighters were typically equipped with a single plasma cannon and J-class with two turrets. The captains of theis freighters, however, would often upgrade the cannons for more powerful ones at the earliest opportunity. To achieve a weapons lock, the target had to be within a nine kilometer range.
The Enterprise NX-01 was equipped with several plasma cannons during the earliest days of its mission but was eventually upgraded with the more advanced phase cannons. Enterprise's Shuttlepod 1 was equipped with dual plasma cannons. These plasma cannons had a range of less than ten kilometers.