First check the printer cable. Obviously a bad printer cable will cause this error. If the cable is good, check the following:
All 390, 880 & 890 series densitometers (and most other models as well) can be configured to monitor their "Pin5" handshake line. There are three possible settings for this option: "CTS", "Busy", & "Off". If the densitometer is being used with a printer, it is essential that this option be set correctly. If no printer is attached, then the option should be set to "Off". Here's how handshake works - -
Densitometers can send data to printers far faster than the printer can process and print. Thus the printer's input buffer will almost certainly become full if (for example) a graph is being processed. The handshake system exists to prevent buffer overflow.
A printer can directly control the voltage of a line it calls RTS (Ready to Send). When the printer is able to accept data (it has room in its input buffer) it will enable this line. The RTS line of the printer is connected to the CTS (Clear to Send) line of the X-Rite densitometer. If the densitometer has been configured to monitor this line, it will ONLY send data when the RTS is enabled. The instant the RTS line is dropped, transmission is suspended. The densitometer will then dutifully wait with all remaining data until again the line becomes active. As the printer catches up - it will again activate the RTS signal. Much like a traffic signal, the RTS/CTS pair controls traffic between the two devices.
X-Rite densitometers expect the printer to be ready at the beginning of a print sequence. They expect the RTS signal (or a green light - If you prefer). If the densitometer has been configured to monitor the handshake line and it is initially off (red light), the warning is immediately sounded in the form of a beep and a display message. Printer is either [a] turned off, [b] out of paper, [c] off line, or [d] not connected.
"Busy" (rather than "CTS") monitors the same line but for opposite signal polarity. Said another way, it interprets a "red" light to be "green" and vice versa. Some printers (or computer software interfaces) require this opposite configuration.
If nothing is attached - - "Off" is the best choice and is guaranteed to make the warning go away.