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HP 7550A and 7550A Plus Plotters - Input/Output (I/O) Buffer Overflow (Error 16)

Issue:
When sending a print job from the software it starts to plot then displays I/O Buffer Overflow (Error 16). The plotter will either stop completely or start to plot strange characters, lines or exhibit other random behavior.
Cause
I/O Buffer Overflow indicates that the plotter's buffer overflowed. When the buffer was almost full, the plotter signaled the Central Processing Unit (CPU) to stop sending data (dropped DTR line voltage on pin 20 for hardware handshake or the plotter sent an XOFF character for software [XON-XOFF] handshake), but the CPU continued to send data and the buffer overflowed.
Several things could cause this problem:
  1. Software is using software handshaking (XON-XOFF) handshake while the plotter is set to hardware handshake. Try setting the plotter's serial handshake to XON-XOFF through the front panel. If possible, try a different software package.
  2. The plotter may be connected to a network or a switching device. See if a direct connection works.
  3. The cable may be pinned out incorrectly. The serial I/O on the computer would not be able to detect the change in Data Terminal Ready (DTR) voltage; consequently, it would continue sending data to the plotter, even though the buffer is overflowing.
WORKAROUND: Try decreasing the baud rate on the plotter and in the software so that data is sent at a slower speed. If lowering the baud rate increases the amount of the plot that comes out, the cable is probably failing. Try an HP manufactured cable (9-pin or 25-pin).
  • If in possession of an HP 7550A Plus plotter, try using parallel instead of serial.
If none of the above steps provide the solution, the problem is with the interface port on either the PC or the plotter.
Changing the Serial Menu
HP 7550A Plotter
While pressing the ENTER button, press the NEXT DISPLAY button. The LCD should then read:
HP-IB
Monitor
Standard
Serial
Press the button beneath Serial to access serial settings. Press NEXT DISPLAY to access:
Data Flow
Bypass
Handshake
After checking those settings, press NEXT DISPLAY to access:
Duplex
Parity
Baud
After checking all the settings, simultaneously press the ENTER button and NEXT DISPLAY buttons, then press the NEXT DISPLAY button again.
HP 7550A Plus Plotter
Press the NEXT DISPLAY button until the LCD shows Serial in the lower right corner. Press the F4 button to access serial settings:
Dataflow
Baud
Parity
Handshake
After checking those settings, press NEXT DISPLAY to access:
Bypass
Monitor
Duplex
Autodisc
After checking all the serial settings, press the Next Display button twice to return to the default display.

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