Hi,

Perhaps someone can shine some light on this. I have a script group dl'ed for the FBW A320. The script for the EXT PWR Korry switch (which I want to use as an example) looks like this and works just fine - I just don't understand how:

(L:A32NX_EXT_PWR_AVAIL:1,·Number)·(L:A32NX_OVHD_ELEC_EXT_PWR_PB_IS_ON,·Number)·!·and·(L:A32NX_OVHD_INTLT_ANN,·Number)·0·==·(L:A32NX_ELEC_DC_2_BUS_IS_POWERED,·Bool)·and·or·1·and·1·and·(L:A32NX_OVHD_INTLT_ANN,·Number)·2·==·if{·0.1·}·els{·1·}·*
(>L:ExtPwrSeq1)

(L:A32NX_EXT_PWR_AVAIL:1,·Number)·(L:A32NX_OVHD_ELEC_EXT_PWR_PB_IS_ON,·Number)·and·(L:A32NX_OVHD_INTLT_ANN,·Number)·0·==·(L:A32NX_ELEC_DC_2_BUS_IS_POWERED,·Bool)·and·or·(L:A32NX_ELEC_AC_1_BUS_IS_POWERED,·Bool)·and·1·and·(L:A32NX_OVHD_INTLT_ANN,·Number)·2·==·if{·0.1·}·els{·1·}·*·
(>L:ExtPwrSeq2)

(L:A32NX_OVHD_INTLT_ANN,·Number) is the position of the test switch: TEST = 0 / BRT = 1 / DIM = 2. The rest should be self explaining.

(L:ExtPwrSeq1) and (L:ExtPwrSeq2) are variables which describe the status of the annunciator, depending on GND PWR and EXT PWR status. (1) controls the AVAIL light, (2) the ON light.

0 + 0 = OFF

1 + 0 = AVAIL

0 + 1 = ON

With GND PWR available (1), EXT PWR off (0), Test switch BRT (0), DC bus not powered (0), the upper half of the script up to the "if" would look like this:

1 0 not and 0 0 == 0 and or 1 and 1 and 0 2 ==

The result is 0 which triggers the els action and sets (L:ExtPwrSeq1) to 1 (AVAIL light is on).

What I don't understand in the order this script is being processed. According to what I found RPN takes two operands and uses the operator on these two. Except for the negation (not), which is performed directly on the preceding operand.

Which brings me to this:

1 1 and 0 0 == 0 and or 1 and 1 and 0 2 ==

And next:

1 0 0 == 0 and or 1 and 1 and 0 2 ==

Here I'm lost. Three operands and a == operator. And then "and or" right behind each other. What am I missing?

Boris

 

What you're missing is the concept of an RPN stack.  Each entry is "pushed" onto the top of the stack.  A dual-operand operator will remove ("pop") the top two elements off of the stack, perform the operation, and push the result back onto the top of the stack.  Here in my explanation, x y z is a stack with z being the topmost (most recent) entry, x being the bottom. 

So 1 0 0 "pushes" three entries onto the stack (1 0 0, with 1 at the bottom, the last zero at the top), then == "pops" the top two (rightmost) entries as operands (0 and 0) does an equality comparison, and pushes the result (1 representing true) back onto the top of the stack, leaving 1 1 on the stack.

Then the 0 is pushed onto the top of the stack (now 1 1 0), the "and" operator pops the top two entries as operands (0 and 1), does a logical "and" yielding a result of false/0 and pushes that back onto the stack, leaving 1 0 on the stack.

Then the "or" pops the remaining two entries off the stack as operands (1 and 0) and performs a logical "or" yielding a result of 1/true, and pushes that back onto the stack leaving the stack with the single entry of 1

Then 1 is pushed onto the stack (now 1 1), the logical "and" pops the two values and pushes the result 1/true back onto the stack, leaving it with a single entry of 1

Then another 1 is pushed onto the stack (now 1 1), the logical "and" pops the two values and pushes the result 1/true back onto the stack, leaving it with a single entry of 1

Now 0 then 2 are pushed onto the stack, (now 1 0 2), and the final == pops the 2 and 0, does a comparison, and pushes the result 0/false onto the stack, leaving you with 1 0 left on the stack for the subsequent "if" test.

 

1 hour ago, borisvp said:

1 0 0 == 0 and or ...

 

 

                 (0)           (0)
          (0)    (0)    (1)    (1)    (0)
-> (1) -> (1) -> (1) -> (1) -> (1) -> (1) -> (1)
push  push   push    ==    push    and    or
 1     0      0             0

etc.