¿Un FB con la variable bar3? La interfaz se queda grabada en el DB de instancia — Transcript

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1. 0:00You open a compressor block and its
2. 0:01status is called bar 3.
3. 0:03It works, but nobody knows what it means
4. 0:05without tracking the whole program.
5. 0:07There is a test I run on any PLC program
6. 0:08when I want to know if the person who
7. 0:10made it knew what they were doing. I
8. 0:12open a random FB without looking at the
9. 0:14header comment, without asking anyone,
10. 0:17and I look only at the interface, the
11. 0:19inputs, outputs, and in-outs. If that's
12. 0:22enough to know what the block does,
13. 0:24someone thought of me before writing it
14. 0:25. If I see in1, IN2, in3, AUX, A2, bit 7
15. 0:29, that programmer didn't structure
16. 0:32anything; they just made it work. The
17. 0:35thing with Siemens PLCs is, when you
18. 0:38program an FB in Portal, the interface
19. 0:40you declare as input, output, in-out, or
20. 0:42static is not just an internal detail.
21. 0:45It gets etched into the instance DB
22. 0:47forever with those exact names, even if
23. 0:49the variable is named differently in the
24. 0:51call.
25. 0:52That is, those names will be inherited
26. 0:54by anyone who calls that block after you
27. 0:56, and they will also be seen by anyone
28. 0:58reading the instance DB from the HMI or
29. 1:00a watch table while trying to understand
30. 1:03why a machine won't start at 3 a.m. I've
31. 1:05opened projects with compressor FBs
32. 1:06where the equipment status was in a
33. 1:08variable called barres barres of type
34. 1:10int.
35. 1:12No idea what each number meant without
36. 1:13going to the segment where it was
37. 1:15written. And that is where it really
38. 1:17grinds my gears. The program worked
39. 1:19perfectly, but that programmer didn't
40. 1:20care in the slightest about who would
41. 1:22touch that code later, not even
42. 1:24themselves in 6 months 'time. My rule,
43. 1:26which I always apply when declaring an
44. 1:28interface, is that the name must say
45. 1:30what it is, not how it is used.
46. 1:32Compressor status instead of bar 3.
47. 1:35Pressure setpoint bar instead of SP1.
48. 1:38If necessary, make it longer but
49. 1:40readable. If an input is a case-type
50. 1:42status code, like the stage variable I
51. 1:44use in my equipment FBs where a number
52. 1:46between one and 3 tells you if the
53. 1:48compressor is available, I state it in
54. 1:50the comment on that same interface line
55. 1:52; not in a separate PDF, not in the head
56. 1:54of the person who wrote it. And I always
57. 1:56separate the physical input from the
58. 1:58process variable. I never put a direct
59. 2:00percentage sign inside the logic of an
60. 2:02equipment FB. I go through the mapped DB
61. 2:04first, and the FB receives the variable
62. 2:06with a process name.
63. 2:08I do this because the day someone
64. 2:09replaces that sensor or changes its
65. 2:11physical address, they shouldn't have to
66. 2:13touch a single line of business logic.
67. 2:15Anyone can get an engine to start by
68. 2:16trial and error. But having the next
69. 2:18person who opens your project not need
70. 2:20to call you—that’s a different story.