Posted 26 Feb 2020 hello everyone , I am involved in the development of an industrial product. It has a 24 V On / OFF output signal which will be read by a PLC digital input. It could be Allen Bradley / Siemens . Can anyone tell me what would be the approximate values of the internal pull ups and pull downs? where can i find this? Thank you for all the help. Regards Sha 1 person likes this Share this post Link to post Share on other sites
Posted 26 Feb 2020 you can read the specs in the datasheets. for example product X may say that at 24V connected to input current is 3.5mA. then use Ohms law 1 person likes this Share this post Link to post Share on other sites
Posted 26 Feb 2020 To panic's point, it depends on platform and module type. Once you know that, then refer to the datasheets to ensure your device fits the specs. Do you wire PNP or NPN. Generally, you shouldn't have to install pull up or down resistors. 1 person likes this Share this post Link to post Share on other sites
Posted 26 Feb 2020 Thanks a lot for your responses. I thought the digital inputs have internal pull up or is that installed externally? About wiring, Please see attached image. My understanding is that R3 is internal and I would like to find the value of that. I looked at Siemens S7 datasheet. Based on the DO numbers, R3 would be 8k. Voltage simulations shows PLC input is not getting close to 0 V in low state. I am concerned that the threshold wont be met and be detected by the PLC as a low. Share this post Link to post Share on other sites
Posted 27 Feb 2020 you assume wrong.... circuit you show model of an input card for NPN devices. This is common in low power electronics where risk of injury is non existent (phones, fax machines, etc.) but those are not same class of devices discussed here and don't get same scrutiny. In industrial automation NPN is used as standard in Japan. Most of the world uses PNP. If you are looking for product with NPN configuration in Europe or NorhtAmerica, you will find them but they will be very few since not common. So for most of the world (including AB and Siemens) your circuit would need to be upside down and resistor would be pull-down, not pull up. And since you show 24V and posted this in industrial forum, it is important to add that your circuit would still be unacceptable even from NPN perspective because you grounded negative side instead of positive. Grounded rail is common or reference potential. All switching is on the "live" side (24V or phase if AC) due to regulations. When working with NPN type I/O in industrial automation, positive terminal of PSU is grounded and therefore 0V. negative terminal is "-24V" and it is live. and switch, fuse, etc would be on the live side. Some companies are from Japan and insist on NPN whereever they are. For example Honda and Toyota are among them. Honda in North America uses NPN for I/O, but for safety circuits, they use separate power supply and western style safety devices because those are more common. (NPN type OSSD exists but it is just too rare here). More over, do you need just one output or multiple I/O? If you are developing devices with I/Os for industrial automation, why not look at products meant to be used for this? Yes they cost few $ more but they already contain TONS of features expected from today's products. And i am not talking about hardware specs alone, this also goes to approvals. And you need approvals if you want to marked industrial product, or every failure will be lawsuit with your name on it. so for North America you want at least UL/CSA and it starts not just with components that already have such approvals but also design practices such as PCB layout, spacing etc. 2 people like this Share this post Link to post Share on other sites
