fig6 a flow chart shown in fig7 indicates the flow of data communication with the flash unit the data communication with the data back conducted in the step f5 is omitted as it is similar to that with the flash unit fig3 shows a timing chart of the data communication with the mounted flash unit referring to fig7 steps f52 f54 maintain the contact b1 at the l level for a predetermined period a c in fig3 and then leave said contact open thereby enabling input at this point a step f55 discriminates the level of said contact b1 and if it is in the l level the flash unit 101 is discriminated as mounted and the data communication is started if the flash unit is not mounted the contact b1 assumes the h level because of the incorporated pull up resistor so that the sequence proceeds to a step f65 for clearing received data from the ram in the flash unit 101 if a step f111 in fig8 identifies the l level state of the contact s1 a step f112 shifts the contact s1 to the output mode and produces an l level signal b in fig3 thereby indicating the enabled state of data communication to the camera 1 and awaiting clock signals and data of a first byte from the camera when the sequence proceeds to a step f56 in fig7 the camera 1 sends the data of the.