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In this paper, the physical layer of Ethernet CSMA/CD of HUB was designed with VHDL. The protocol of physical sub-layer of CSMA/CD was analyzed and the control circuits in HUB were designed with finite state machine (FSM). The whole system included port-controller, arbiter, clocks MUX, FIFO, core-controller and symbol generation and MUX. The functions of CSMA/CD were realized mainly by port-controller. Some parts of source programs were given out. The design was simulated in MAX+PLUS II tools under the clock frequency of 25 MHz. The simulated results demonstrate that the functions of physical sub-layer of CSMA/CD are realized.