What thing is this?
You have to do the work, but we’ll help. What is your obstacle at the moment?
In general -
You need to create a serial Bridge thing. This represents your slave device, you will need to configure it with your comms details e.g. port, baud, id.
You may need to take steps to make your serial port consistently available to openHAB, depending on your host system, see the binding docs.
As a child of that Bridge, create a poller Bridge thing. This specifies one or more registers in a block to read-poll, so you’d need to configure it with register addresses, type, etc.
(Later you can add more pollers for different register blocks)
As a child of the poller, create a data Thing. This describes how to interpret the raw registers e.g. integer or float, etc, so you need to know how your slave populates them.
(Later you can add more data Things to each poller)
(Later you can add write details for any writable registers)
The data Thing has several channels available of different types, link the type you want to an appropriate Item type.
If you’ve already used some test software successfully, you already have all the details to set up your first Item to read. Let us know how you get on.