Patent ID: 8626050

Claim:
An image forming apparatus connected to a detachable sheet feeding unit configured to be able to feed sheets at intervals, comprising: an image forming unit configured to form an image on a sheet; an transmitting unit configured to transmit a feeding command to the detachable sheet feeding unit via a signal line; a second sheet detecting unit located at downstream of a first sheet detecting unit in a sheet conveyance direction, configured to detect a sheet fed by the detachable sheet feeding unit; a receiving unit configured to receive a detection result of the first sheet detecting unit via the signal line; a controller configured to: if the said detachable sheet feeding unit feeds the first sheet and the second sheet to make an interval between a trailing edge of the first sheet and a leading edge of the second sheet, when the first sheet and the second sheet with the interval has passed the first sheet detecting unit and the detection result of the first sheet detecting unit indicates that the first sheet detecting unit is still detecting a sheet and the second sheet detecting unit does not detect the interval, stop an image forming of the image forming unit; and when the detection result of the first sheet detecting unit indicates that the first sheet detecting unit is detecting a sheet and the second sheet detecting unit detects the interval, continue the image forming of the image forming unit; the image forming apparatus further comprising; a first counter configured to start counting time in responding to transmission of the feeding command and continue counting the time until the first sheet detecting unit detects a sheet; a second counter configured to start counting time in responding to transmission of the feeding command and continue counting the time until the second sheet detecting unit detects a sheet; a sheet feeding error determining unit configured to determine a sheet feeding error when the detecting result of the first sheet detecting unit continuously indicates no sheet until the time counted by the first counter exceeds a first threshold time; and a retention jamming determining unit configured to determine a retention jamming when the time counted by the second counter exceeds a second threshold time and the second sheet detecting unit does not detect the interval, wherein the controller stops the image forming when the sheet feeding error or the retention jamming has occurred.