This document applies to all versions of AIX.
The information in this document applies to those situations where a queue goes to a DOWN state after having printed only a portion of the submitted job, or where there is otherwise some data loss, or where output from the cat command is redirected to the device file and printed output shows data loss.
All serial printers require some means of regulating the flow of data to and from the system to which they are attached. The suspension and resumption of data flow is necessary in a variety of circumstances. For example, the host to which the printer is attached must be signaled to stop sending data for the following reasons:
XON/XOFF refers to the characters sent to resume and suspend transmission respectively. As the operating system must interpret these characters, this method is termed "software" flow control. When the printer buffer nears capacity, the printer sends an XOFF character to the system to signal it to suspend data transmission. After most of the buffer contents have printed, the printer sends an XON character to the system to signal it to resume data transmission.
DTR refers to the Data Terminal Ready signal. The printer DTR pin is wired to the system DCD (Data Carrier Detect) pin. As the serial port regulates this kind of flow control, it is referred to as "hardware" flow control. When the printer buffer nears capacity, the printer drops DTR, at which point the system no longer has the signal that indicates the presence of the printer and the serial port suspends data transmission. When most of the contents of the the printer buffer have printed, DTR is raised, causing the serial port to detect the presence of the printer and resume data transmission.
NOTE: DTR pacing is not a possibility for all printers.
This is another kind of "hardware" flow control. The RTS/CTS method uses Request To Send (RTS) - Clear To Send (CTS) signaling. It is commonly used for terminals and modems. The few brands of printers that use this method drop RTS when the printer buffer nears its capacity. The printer RTS pin is wired to the system CTS pin. The AIX lp driver, however, does not not allow for this kind of flow control; the lp driver requires that CTS be high at all times. When used with AIX these printers must use the tty driver, or a custom cable must be built.
NOTE: A tty driver must be used where the printer is limited to this kind of flow control and standard cabling is used.
SSD is a kind of "hardware" flow control used by very few printers. AIX does not support this method. For use with AIX, printers intended for SSD flow control must have the SSD pin wired to the system DCD and the lp driver must be configured for DTR flow control.
A "full spec" cable (typically 8 wires) is required for "hardware" flow control (that is, flow control other than XON/XOFF). Modems, multiplexors ("muxes"), and so on that are used on the connection between the printer and the RS/6000 serial port do not allow for the signaling required by hardware flow control. In these situations, XON/XOFF flow control must be used.
lptest generates output in a "barber pole" pattern that makes data loss readily apparent. To redirect the output of lptest to the lp device, execute:
lptest > /dev/lp#
# is the number of the lp device file.
Where data loss (printed output missing characters) is occurring, consider the following resolutions: