Grass Valley PDR v.2.2 - Manual (Page 319)

System Control Returns
Profile Family 301
System Control Returns
ACK
ACK
Acknowledgment.
Packet format: 20 41 cs
where: cs = checksum
NAK
NAK; {Time Out, Frame Error, Overrun, Parity Error, Checksum Error,
Undefined Command}
Negative acknowledgment. Any or all errors may be returned.
Packet format: 21 4E er cs
where: er = error bits (one-byte value)
cs = checksum
The error bits are defined as:
bit0: Undefined command
bit1: (Unused)
bit2: Checksum error
bit3: (Unused)
bit4: Parity error
bit5: Overrun
bit6: Frame error
bit7: Timeout