A surgical instrument holder includes a carriage, a housing, and a drive assembly. The carriage is configured for engagement to a surgical robotic arm and for supporting an instrument drive unit. The housing extends from the carriage and defines a channel. The drive assembly includes a pulley, a belt, and an annular member. The pulley is rotatably disposed within the housing and in operable engagement with a motor of the carriage such that actuation of the motor rotates the pulley. The belt is rotatably disposed within the housing and in operable engagement with the pulley such that rotation of the pulley effects rotation of the belt. The annular member is disposed within the channel of the housing and configured for non-rotatable receipt of an instrument drive unit. The annular member is in operable engagement with the belt such that rotation of the belt effects rotation of the annular member.