In train printers utilizing a ''''universal'''' or ''''soft'''' translate buffer wherein the code representations for the characters on the type train may be changed or reprogrammed by reloading the translate buffer, an efficient way of determining whether each character code being received represents a character in the type train being utilized is to use the character code as an address to a read/write memory which stores a logic 1 in all locations that are addressed by valid character codes and a logic 0 in all other or invalid character code locations. The output from this memory, each time it is addressed is utilized to activate a selector to pass either the incoming character code or an invalid character code from an invalid character code register, depending on whether the memory output was a logic 1 or a logic 0, respectively.