A PLC is a programmable logic controller.
A silicon integrated circuit is very unlikely to have quartz crystal inside the package. What it does have is an onboard oscillator that may or may not be factory trimmed to some specified level of accuaracy.
In asyncronous communications plus or minus 2% on the baudrate should work. For example at 9600 baud anything from 9408 to 9792 has a pretty good chance of working. I've personally used 9535 and 9615 as actual baudrates with no fallouts over several thousand units.