> For the complete documentation index, see [llms.txt](https://help.security.gallagher.com/knowledge-hub/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.security.gallagher.com/knowledge-hub/products/cc-hardware/cabinets-and-power/seperate-power-supplies.md). # Secondary Power Supplies When powering Gallagher devices from a secondary power supply, other than the one powering the controller. ## The three common connections When wiring Gallagher devices (HBUS, GBUS, Local BUS and Cardax IV) there are 3 connections that **simply must be common** to the controller they are connected to: * A and B for the buses * White and Blue wires for Cardax IV * Last but not least, the 0 V or ground (Black) wires Power can be supplied via a 2nd local Gallagher power supply, or a 3rd party power supply. > ⚠️ **The grounds must be common (connected) in this case**, whichever power supply is used. ## The RS485 grounding rule The 485 BUS is an international standard that **requires the Ground to be present**. In some circumstances the bus will work with no ground, but that is **only** because the 485 devices are at almost the same common reference voltage level (ground) and appear to work. However, when that changes, the communications will become intermittent and unreliable. **This is one of the most common problems with Fence Controllers.** > ⚠️ **Chasing intermittent fence controller comms?** Check the ground bond first, before biasing or termination. ## Wiring examples In the image below, power for the Controller, the 8 In 2 Out Door Module on the left, and the T11 reader is supplied by the power supply on the left. The power for the 8 In 2 Out Door Module on the right and the T20 is supplied from the power supply on the right. ![Separate power supplies, wiring example 1](https://328339335-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F6bM8WhlBE9BYizPWJaPN%2Fuploads%2Fgit-blob-1ce93fb531bc67df4322ca7c8f749a43b2c691de%2FSECHUB-PSU-SEPARATE-01-external-power-example-01.jpg?alt=media) The common item between the two power systems is the ground, and **this must be present to have reliable communications**. You can substitute the T20 for a 3rd party reader and, **as long as** the 8 In 2 Out Door Module is a 4 In 2 Out Door Module Wiegand variant, it will work the same. This is another example of the requirement of the common 0 V connection. ![Separate power supplies, wiring example 2](https://328339335-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F6bM8WhlBE9BYizPWJaPN%2Fuploads%2Fgit-blob-9ce3473794b4e35aec83e539841e56fb843ae7c1%2FSECHUB-PSU-SEPARATE-02-external-power-example-02.jpg?alt=media) ## A common mistake Another common mistake is to think that the Fence Controllers require only the A and B connections. This is a mistake that is very common. The RS485 standard requires a common ground for the serial interface to work *reliably*. ![Separate power supplies, wiring example 3](https://328339335-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F6bM8WhlBE9BYizPWJaPN%2Fuploads%2Fgit-blob-13e1f1a735985630d2d162a775f4f43c34e31f58%2FSECHUB-PSU-SEPARATE-03-external-power-example-03.jpg?alt=media) *** 🔗 **Related** * [Gallagher Power Supplies - How to Guides](/knowledge-hub/products/cc-hardware/cabinets-and-power/gallagher-power-supplies-how-to-guides.md). The 8 A and 3.5 A Gallagher supplies, their capacities and their monitoring outputs. * [Cabinets and Power](/knowledge-hub/products/cc-hardware/cabinets-and-power.md). The cabinet and power supply range. * [Input and Output (IO) Devices](/knowledge-hub/products/cc-hardware/input-and-output-io-devices.md). The door modules shown in the wiring examples. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://help.security.gallagher.com/knowledge-hub/products/cc-hardware/cabinets-and-power/seperate-power-supplies.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.