R740 8X2.5 chassis does not support on-board SATA. It only support PERC configuration. As it is not supported there is no cable for same.  You can refer below link for supported configuration for various R740 chassis. [URL]   
Hi all, I am searching for the cables needed to connect the on-board SATA controller connectors to 8X2,5" backplane with no luck. Could some one help? Regards,
R740 8x2,5" on-board SATA to backplane cable