I wasn't sure if the printer would just ignore the command with the lowercase terminator. So I tried printing the following commands to a Kyocera FS-4020DN and a SHARP MX-M503N the output from these two printers looked fine.
Ec*p400xEc*p2500yEc&a90pEc(s0SI'm 90 degrees
According to the PCL specification a terminator is "Any character from the ASCII table within the range 64-94 decimal (\ @ " through \ ^ "). This character specifies the parameter to which the previous value field applies. This character terminates the escape sequence".
I was wondering if someone could explain why the two printers and PCL Viewer I'm using interprets these invalid commands correctly.