Clears received characters in the buffer of MACRO.
Characters received from the host are transferred to MACRO. MACRO stores the characters in the buffer.
Character-reading commands, such as the "wait" command, read out them from the buffer.
Characters in the buffer are kept until character-reading commands process them or the buffer overflows or the flushrecv command clears the buffer.
The "flushrecv" command can be used to avoid unexpected results of character-reading commands caused by old characters in the buffer.