一开始做下机的时候没有想到强制下机。有一个卡号我忘记下机了，结果cash成为负数了。所以就看师哥师姐的博客，看到了强制下机。弄强制下机是非常有必要的，在网吧上网的时候，如果一直上机  ，卡里余额为0甚至为负数，还在无止境的上机，商家不得赔钱？

下机流程图：

这是下机的流程图。动态下机利用timer控件，展示一下代码：

'判断是否有学生上机
    txtsql = "select * from online_info "
    Set mrcon = executeSQL(txtsql, msgtext)
    If mrcon.EOF Then
        Timer2.Enabled = False  'timer2控件不可用
        Exit Sub
    Else
        mrcon.MoveFirst             '移至第一条记录
    End If
Rem:把上机卡号定义为一个数组
    ReDim j(mrcon.RecordCount) As String
    For i = 0 To mrcon.RecordCount - 1
    j(i) = Trim(mrcon!cardno)
    cardno = j(i)
Rem:查询表，online_info表，basicdata_info表，student_info表，line_info表
    txtsql = "select * from online_info where cardno='" & Trim(cardno) & "'"
    Set mrcon1 = executeSQL(txtsql, msgtext)

    txtsql = "select * from basicdata_info "
    Set mrcba = executeSQL(txtsql, msgtext)

    txtsql = "select * from student_info where cardno='" & Trim(cardno) & "'"
    Set mrcst = executeSQL(txtsql, msgtext)

    txtsql = "select * from line_info "
    Set mrcli = executeSQL(txtsql, msgtext)
    Rem:记录上机时间
    s = Trim(mrcon.Fields(6))
    q = Trim(mrcon.Fields(7))
    Rem:计算上机时长
    mintime = DateDiff("n", q, Time)
    mindate = DateDiff("n", s, Date)
    ztime = Val(mintime) + Val(mindate)
    Rem:总的上机时间首付大于准备时间
    If ztime < mrcba!preparetime Then
        txtctime.Text = 0
        txtcmoney.Text = 0
        txtcash.Text = mrcst.Fields(7)
    Else
        Rem:判断用户类型进行收费(固定用户or临时用户)
        Rem：消费时间=上机时间-准备时间
        ztime = ztime - Trim(mrcba.Fields(3))
        txtcmoney.Text = ztime
        Rem:判断消费时间是否超过最低消费时间
        If ztime < Trim(mrcba.Fields(3)) Then
            ztime = Trim(mrcba.Fields(3))
            txtctime.Text = Trim(mrcba.Fields(3))
        Else
            ztime = ztime
        End If
        Rem:判断是否是固定用户，计算消费金额
        Rem:固定用户
        Dim pmoney As Currency      '定义消费金额
        If Trim(txtType.Text) = "固定用户" Then
        Rem:round函数
            pmoney = Format(Round(ztime / mrcba.Fields(2), 1) * mrcba.Fields(0), "0.00")
        Else
            '临时用户
            pmoney = Format(Round(ztime / mrcba.Fields(2), 1) * mrcba.Fields(1), "0.00")
        End If
        Rem:动态计算余额
        dmoney = Format(mrcst!cash - pmoney, "0,00")
        txtcmoney.Text = Format(pmoney, "0.0")  '
        txtcash.Text = Format(dmoney, "0.00")
        mrcst!cash = Format(mrcst!cash = pmoney, "0.00")
        Rem:更新student
        mrcst.Fields(7) = Trim(txtcash.Text)
        mrcst.Update

        If Val(txtcash.Text) > 0 And Val(txtcash.Text) < Trim(mrcba.Fields(5)) Then
            MsgBox j(i) & ",您的余额已经不足" & mrcba.Fields(5) & "元，请您尽快充值！", vbOKOnly + vbExclamation, "提示"
        End If
        Rem：余额为0时，会强制下机，更新表
        If mrcst.Fields(7) <= 0 Then
            mrcst.Fields(7) = 0
            mrcst.Update
            mrcon1.Delete
            mrcon1.Update
            MsgBox "卡号" & j(i) & "余额不足，即将下机！", vbOKOnly + vbExclamation, "提示"
            Rem:更新line表
            mrcli.Fields(8) = Date
            mrcli.Fields(9) = Time
            mrcli.Fields(12) = mrcst.Fields(7)
            mrcli.Fields(10) = ztime
            mrcli.Fields(11) = pmoney
            mrcli.Update
        
      End If
     End If
    Next i

一定要多想想，为用户着想，结合实际。