综合场景例程 2：恒压供水（一用一备轮换 + 压力回差）

这一页只讲这一个例程。母页 b-05-integrated-scenarios 里留着例程 1，其余例程各自一页、在那一页的目录里点。

场景：两台泵维持管网压力，压力低开泵、高停泵（带回差），每次启动换另一台，故障的那台自动跳过。

软元件

软元件	作用
D600	管网压力（kPa，300 = 0.30MPa）
M500	运行需求
D610	轮换标志
M511 / M512	1 号 / 2 号故障
Y0 / Y1	1 号泵 / 2 号泵
Y20	故障指示

块图（照着拖即可）

对应的 ddc.json（点「生成 JSON」得到的就是它；也可存成 .ddc.json 导入）

{
  "version": 1,
  "name": "恒压供水",
  "scan_ms": 200,
  "lines": [
    {
      "op": "assign",
      "target": {"op":"ref","device":"M","addr":500},
      "expr": {
        "op": "rs",
        "id": "rs_pump",
        "args": [
          {
            "op": "cmp",
            "cmp": "<",
            "args": [{"op":"ref","device":"D","addr":600},{"op":"const","value":300}]
          },
          {
            "op": "cmp",
            "cmp": ">",
            "args": [{"op":"ref","device":"D","addr":600},{"op":"const","value":450}]
          }
        ]
      }
    },
    {
      "op": "if",
      "cond": {"op":"edge","edge":"rise","id":"e_alt","args":[{"op":"ref","device":"M","addr":500}]},
      "then": [
        {
          "op": "assign",
          "target": {"op":"ref","device":"D","addr":610},
          "expr": {
            "op": "arith",
            "a": "-",
            "args": [{"op":"const","value":1},{"op":"ref","device":"D","addr":610}]
          }
        }
      ],
      "else": []
    },
    {
      "op": "assign",
      "target": {"op":"ref","device":"Y","addr":0},
      "expr": {
        "op": "and",
        "args": [
          {"op":"ref","device":"M","addr":500},
          {
            "op": "and",
            "args": [
              {
                "op": "cmp",
                "cmp": "==",
                "args": [{"op":"ref","device":"D","addr":610},{"op":"const","value":0}]
              },
              {"op":"not","args":[{"op":"ref","device":"M","addr":511}]}
            ]
          }
        ]
      }
    },
    {
      "op": "assign",
      "target": {"op":"ref","device":"Y","addr":1},
      "expr": {
        "op": "and",
        "args": [
          {"op":"ref","device":"M","addr":500},
          {
            "op": "and",
            "args": [
              {
                "op": "cmp",
                "cmp": "==",
                "args": [{"op":"ref","device":"D","addr":610},{"op":"const","value":1}]
              },
              {"op":"not","args":[{"op":"ref","device":"M","addr":512}]}
            ]
          }
        ]
      }
    },
    {
      "op": "assign",
      "target": {"op":"ref","device":"Y","addr":20},
      "expr": {
        "op": "or",
        "args": [{"op":"ref","device":"M","addr":511},{"op":"ref","device":"M","addr":512}]
      }
    }
  ]
}

要点

• 启停控制的启动/停止孔各接一个阈值，天然就是带回差的压力开关（300 开、450 停），比自己写两条比较清楚。
• 回差按泵的最少连续运行时间定：太小（如 20kPa）会导致频繁启停，电机和止回阀都受罪。
• 压力建议用 kPa 整数（D 寄存器是 16 位整数），别把 0.30MPa 直接存进 D。

验证：D600 写 200 → M500 = 1、Y0 或 Y1 动作；写 500 → 停；再写 200 → 换另一台；把 M511 置 1 → 只会是 Y1 动作。

注意事项

• 压力传感器断线（值变 0）→ 一直满足"低于下限"，泵不停：必须有断线保护（见例程 5）。
• 回差设成 0：泵在临界点每分钟启停好几次。
• 两台泵没做故障屏蔽：故障泵照样被选中，压力一直上不来。