Latches

Latches were first introduced into EXRAIL as a way of latching a sensor value or pushbutton on, so that once detected it could be checked later and not suffer from bouncing. This use is comparatively rare and has generally been superseded by the more recent ONBUTTON event handler.

Latches were also used as a way of setting a flag that could be read later, for example to remember whether a light had been switched on by a ROUTE so that the next call to the same route could switch it off again.

Unfortunately, the LATCH implementation was designed in a limited way to support tiny processors like the UNO or NANO so the latch numbers were limited and could only apply to on-board pins and not extended VPINs. It became common to use latch numbers above the real pin numbers but below the artificial software limit (63 on a UNO, 255 on bigger CPUs) for various flags in routes and automations.

Because of these limitations, it is recommended that you use the much simpler flags feature which provides virtual VPINs that can be set, reset and tested without the limitations of LATCH.

For example:

HAL(Bitmap,800,16) // create flags 800..815 
ROUTE(600,"Shed lights")
  IF(800)  // is the light on? 
     RESET(120)  // lights off
     RESET(800)
  ELSE  // light is off 
     SET(120)  // lights on
     SET(800)
  ENDIF
  DONE

Note that the code cannot use IF(120) to test if the light is on because pin 120 is an OUTPUT pin on a physical device.