Citizen CL-S6621 Technical Manual page 78

Interface
[While receiving PRIME signal]
10~15µsec or more
PRIME
BUSY
Ack
Fault
SELECT
Note: If the PRIME signal width is 10 µsec or less, it is not accepted.
BUSY goes to "High" when the PRIME signal is accepted by the printer.
[Timing relationship between BUSY and ACK]
Center
BUSY
ACK
About 2 µsec
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CL-S6621