NAiS FP Series User Manual page 201

Communication Function 2 General - purpose Serial Communication
When the F159 (MTRN) instruction is executed, the number of bytes received is
cleared, and the address (write pointer) returns to the initial address in the reception
buffer.
When the F159 (MTRN) instruction is executed, the error flag "R9037 or R9047",
reception done flag "R9038 or R9048" and transmission done flag "R9039 or R9049"
go off.
Duplex transmission is disabled while the F159 (MTRN) instruction is being executed.
Check the transmission done flag "R9039 or R9049".
Reception stops if the error flag "R9037 or R9047" goes on. To resume reception,
execute the F159 (MTRN) instruction and turns off the error flag.
Note
Be aware that the reception done flag "R9038 or R9048" changes
even while a scan is in progress.
Example: If the reception completed flag is used multiple times
as an input condition, there is a possibility of different statuses
existing within the same scan. To avoid this, an internal relay
should be substituted at the beginning of the program.
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