Epson C824431 Manual page 20

With the status flag set, and data reception prohibited, the
vacant area of the buffer gradually increases as the printer
continues to print. When the vacant area in the buffer
reaches the specified value (typically 512 bytes; refer to your
printer manual), the flag is reset and data entry is again
enabled.
Note
Reverse Channel pin (No. 11) and DTR pin (No. 20) are
internally connected on the interface board and must have
identical signal levels.
Printer status error
The flag will be set immediately regardless of the remaining
buffer capacity if the printer detects an error.
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