Motorola SYMBOL LS7708 Product Reference Manual page 131

RS-232 Host Interface 6-25
ENQ
When this option is selected, the scanner waits for an ENQ character from the host before transmitting data. If an ENQ is not received
within the Host Serial Response Time-out, the scanner issues an error indication and discards the data. The host must transmit an
ENQ character at least every Host Serial Response Time-out to prevent transmission errors.
ENQ
ACK/NAK with ENQ
This combines the two previous options.
ACK/NAK with ENQ