#604 start of total rework of redstone RTUs for relay functionalitiy

rtu/dev/redstone_rtu.lua

@@ -11,10 +11,14 @@ local digital_write = rsio.digital_write
 
 -- create new redstone device
 ---@nodiscard
+---@param relay? table optional redstone relay to use instead of the computer's redstone interface
 ---@return rtu_rs_device interface, boolean faulted
-function redstone_rtu.new()
+function redstone_rtu.new(relay)
     local unit = rtu.init_unit()
 
+    -- physical interface to use
+    local phy = relay or rs
+
     -- get RTU interface
     local interface = unit.interface()
 
@@ -30,6 +34,12 @@ function redstone_rtu.new()
         write_holding_reg = interface.write_holding_reg
     }
 
+    -- change the phy in use (a relay or rs)
+    ---@param new_phy table
+    function public.change_phy(new_phy) phy = new_phy end
+
+    -- NOTE: for runtime speed, inversion logic results in extra code here but less code when functions are called
+
     -- link digital input
     ---@param side string
     ---@param color integer