In our PLC in general design, we used to use outputs for relay outputs, so you can do enforcement agents contact signal needs, such as some required AC voltage 220V, some need DC voltage, is a passive touch point, etc. In the output of the PLC–contact protection, generally use the RC protection, some protection is not required. PLC sold in markets in General, relay output accounted for a big part, such needs are to meet technicians.
In my design, deep feeling there are many irregularities in this selection to the PLC:
1, contact protection, some of the outputs of the PLC is direct control over the executive bodies at the scene, such as solenoid valves, and in a harsh industrial environment, we cannot fail to consider the line of aging, damaged wire short circuit caused by contact. Once damaged, to change the PLC output relay or changing the contact and modify the PLC program, this is very troublesome for the maintenance personnel.
2, contact waste, the relay outputs of the PLC output are grouped, several are common to the output a single end, so that the output of the form must be the same, otherwise we will be using an external relay transformation. But sometimes the phenomenon, the two points due to take the same action in different lines, or use different sets of two points, or a point, then relay conversion. Make people feel uncomfortable.
3, price problems in PLC‘s specifications, the relay outputs of the PLC is relatively expensive transistor output 20%
We recommend the use of transistor output for PLC, all external relay output, so you can avoid the above problems, maintaining a simple, convenient, and no longer consider modifying the program. In both the design and can reduce the conflict due to different forms of output points to consider, and to facilitate the reasonable distribution of output, so as to achieve calm design.
Transistor output for PLC‘s price is cheaper, even if an external relay or PLC lower prices than direct relay output.