You should define "cheap".
"Daisy chain" does not usually work well with RS-232 devices because they are typically not smart enough to recognize that a command is targeted to a particular device and there are some electronic issues. (I assume that you are dealing with 28 identical, dumb devices.) Traditionally, there is a central controller with multiple RS-232 ports, each port is wired to a single controlled device, the controlled devices are not aware of each other, and there is no electronic interaction between the controlled devices. Any "awareness" is managed by the central controller.
If you have good WiFi coverage in the area, one approach would be to use Global Caché iTach Flex
. This will eliminate the wiring cost and some electronic considerations that could become ugly show stoppers in a clutter of 28 devices scattered over a building.
Each of us will suggest a different central controller and display device, based mostly on our familiarity with a few schemes. My vote would be for a pad/phone based system because they are readily available and there are some inexpensive Apps that can easily handle your scope. While not exactly what you asked for in your original post
, this approach will keep the costs down. If you later want to get into monitoring and alarms, this basic pad/phone approach will not be appropriate.
I know that you sent me a PM. Unfortunately, I am not in the UK. You should be able to find a local precocious teen or a college student who can handle the iTach's and an App.
If these 28 devices also offer control over an Ethernet connection, this can be handled by an App and the only hardware costs would be a pad/phone, network wiring, and (possibly) a larger network switch. Programming could be accomplished in an afternoon. The most difficult decision would be the screen layout, but this might be as simple as 28 ON/OFF buttons with additional "All ON" and "All OFF" buttons.