Dascom 7010 Programmers Manual DMX - Manual (Page 137)

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3 DTR protocol
(a) Conditions for making DTR signal Ready (High)
The following must be required:
･ Printer is on line.
･ Receive buffer has more than 1K bytes available.
Note: When receive buffer has less than 128 bytes available,
DTR signal becomes “Busy (Low)” level and this “Busy (Low)
level is kept until receive buffer has at least 1 K bytes
available.
(b) Conditions for making DTR signal BUSY (Low)
･ Printer is in error.
･ Receive buffer has less than 128 bytes available.