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Checkpointing for Printer Restart (Sequence Number Method)

IP.com Disclosure Number: IPCOM000053159D
Original Publication Date: 1981-Sep-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Maholick, AW: AUTHOR [+2]

Abstract

In computer-connected output printers, when an error is detected by the printer, the host computer will be notified by an appropriate error status indication signal. The host computer will then solicit print job restart recovery information from the printer. Based on the information, the host processor can determine where it must resume transmission in the data stream that was sent in order to restart the job at the beginning of a new sheet of paper corresponding numerically to the page where the error occurred. If the printer is inoperative, the job may actually be restarted on another printer since the information tells the host where to begin retransmission.

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Checkpointing for Printer Restart (Sequence Number Method)

In computer-connected output printers, when an error is detected by the printer, the host computer will be notified by an appropriate error status indication signal. The host computer will then solicit print job restart recovery information from the printer.

Based on the information, the host processor can determine where it must resume transmission in the data stream that was sent in order to restart the job at the beginning of a new sheet of paper corresponding numerically to the page where the error occurred. If the printer is inoperative, the job may actually be restarted on another printer since the information tells the host where to begin retransmission. The host processor insures that the first byte of data retransmitted begins at the specified Request Unit (RU) sequence number and provides to the printer the checkpoint data necessary to resynchronize to the page on which printing is to resume. N pages are bypassed without printing by the printer to avoid duplicating those pages printed without error, from the checkpoint to the page on which the error occurred or a previous page.

Figs. 1 through 4 illustrate possible restart conditions involving data streams that may have transparent data or compressed/ compacted data within them. The recovery point is defined based on the assumption that the printer keeps a remembrance of Request Unit sequence numbers passed in the standard SNA (System Network Architecture) format headers to identify blocks of data.

The first case is that illustrated in Fig. 1. A simple data stream containing no parameterized data is assumed. The checkpoint occurs at some arbitrary byte in the data stream every K pages at the first printed character following the page ejection, as specified by the host processor set checkpoint interval command. In order to inform the host computer where the printer was in the printing job at the time the error was detected, it is necessary to provide identification of the current Request Unit sequence number. This number if maintained in a register which is updated with each new Request Unit header. It is also necessary to provide the position in number of bytes which have elapsed in the job since the start of the current Request Unit. This is defined as the control sequence offset count and is a number maintained in a counter register, that is, incremented with each byte of data printed. In Fig. 1, a checkpoint has occurred at the indicated spot and a Delta(1) exists from the start of the given Request Unit. By providing the control sequence number N and the Delta(1) offset count as a number of bytes actually printed up to the time the checkpoint occurs, the host computer will be informed of where to begin retransmission of the data. The necessary counters and logic circuitry for storing the counts are not illustrated since these are obvious to those skilled in the art or can be implemented in microcode routines....