modelsim仿真的问题

我看夏宇闻老师的《Verilog 数字系统设计教程》第十一章例组合逻辑二一个三态数据通路控制器

代码：

`define ON 1'b1
`define OFF 1'b0

module zhlj2(LinkBusSwitch,outbuf,inbuf,bus,clk);
input LinkBusSwitch;
input[7:0] outbuf;
output [7:0] inbuf;
input clk;
inout [7:0] bus;

wire LinkBusSwitch;
wire [7:0] outbuf;
reg [7:0]  inbuf;
wire [7:0]  bus;

assign bus=(LinkBusSwitch == `ON)  ? outbuf : 8'hzz;

always @(posedge clk) 
	begin
		if(!LinkBusSwitch)
			inbuf<=bus;
	end
	
	
endmodule

仿真代码：

`timescale 1 ns/ 1 ps
module zhlj2_vlg_tst();
// constants                                           
// general purpose registers
//reg eachvec;
// test vector input registers
//reg [7:0] treg_bus;
reg clk;
reg [7:0] outbuf;
// wires                                               
reg LinkBusSwitch;
wire [7:0]  bus;
wire [7:0]  inbuf;

// assign statements (if any)                          
//assign bus = treg_bus;
zhlj2 i1 (
// port map - connection between master ports and signals/registers   
	.LinkBusSwitch(LinkBusSwitch),
	.bus(bus),
	.clk(clk),
	.inbuf(inbuf),
	.outbuf(outbuf)
);


initial                                                
begin                                                  
// code that executes only once                        
// insert code here --> begin 
	//begin
		LinkBusSwitch=0;
		clk=0;
	//end
	forever
		#5 clk=~clk;
 end
 
// --> end                                             
   initial
		begin
 
			
			#20 outbuf=15;
			#20 outbuf=38;
			#20 LinkBusSwitch=1;
			
			#20 outbuf=122;
			#20 outbuf=238;
			#20 outbuf=72;
			
			#20 LinkBusSwitch=0;
			#20 outbuf=55;
			#20 outbuf=0;
			$finish;
		end
	
                                             
endmodule

有一处波形不对，就是outbuf=238时，显示-18.

波形图如下：

请问高手，怎么回事？谢谢！