Select Bit in Shift Register (DirectSOFT)...

Hi guys,

Sorry I haven't had much time to contribute to the forum lately. I still drop in on occasion... :site:

Let's say I have a 32-bit shift register. I want an operator to be able to enter a value from 0 to 31 to select which bit to examine. When the corresponding bit in the shift register is ON, an output will turn on.

Looks like I want to use the 'decode' instruction (DECO), which will take a value from 0-31 and turn on the corresponding bit in the result. Then, take that result and AND it with the shift register. If the matching bit in the shift register is also ON, then the result will have a '1' in that location. Otherwise, the result will contain all zeros. Therefore, if the result is 'greater than' zero, the shift register bit being examined must be ON, so I can turn on my output.

Quicky ladder...

|     ALW_ON               +------------+

|------] [------------+----| LD   V2000 | V2000 Contains a value of 0-31

|                     |    +------------+

|                     |    +------------+

|                     +----| DECO       | Turn on the corresponding bit

|                     |    +------------+

|                     |    +------------+

|                     +----| OUTD V2001 | V2001 and V2002 contain the decoded value

|                     |    +------------+

|                     |    +------------+

|                     +----| LDD  V2001 |

|                     |    +------------+

|                     |    +------------+

|                     +----| ANDD V2003 | V2003 and V2004 contain the shift register bits

|                     |    +------------+

|                     |    +------------+

|                     +----| OUTD V2005 | V2005 and V2006 contain the result of the AND

|                     |    +------------+

|                     |    +------------+

|                     +----| LDD  V2005 |

|                     |    +------------+

|                     |    +------------+

|                     +----| CMPD    K0 | Compare the result to zero

|                          +------------+

|  RESULT > 0

|------] [--------------------------(OUT) If the result of the comparison is greater than zero, turn on the output

For example:

V2000 = 19 Value entered by operator
V2001/V2002 = 0000000000001000 0000000000000000 Decoded value (19th bit is ON)
V2003/V2004 = 0000100010110100 0100010100110010 Current shift register value (19th bit is OFF)
V2005/V2006 = 0000000000000000 0000000000000000 Result of AND. Since result is NOT greater than zero, output is OFF

Shift register advances

V2001/V2002 = 0000000000001000 0000000000000000 Decoded value (19th bit is ON)
V2003/V2004 = 0001000101101000 1000101001100100 Current shift register value (19th bit is now ON)
V2005/V2006 = 0000000000001000 0000000000000000 Result of AND. Since result is greater than zero, output is ON

Am I approaching this correctly? I know that I don't have to store the intermediate results of the accumulator. I just left them in for clarity. I will have a few dozen of these, all looking at the same shift register. Just wondering if there might be a more elegant solution?

I'm using a D2-250-1 CPU



-Eric

I think it's fine. We use the DECO - ANDD in our units.

One application is to test for the presence of an item in a shift register, like yours.

The other is to, on a timed basis, cycle through possibly multiple error bits to be able to display, under PLC control, multiple error conditions - in that case the selector (your V2000) is incremented on a time basis. If the result of the ANDD is ON then a copy of the selector is sent to another V register which acts as the index for a multi-state message display.

Sorry I haven't had much time to contribute to the forum lately. I still drop in on occasion...

Click to expand...

Hello Eric,
Several have wondered what happened to you. I am glad to know that you are still alive and able.

:site:

Thanks for the confirmation, Bernie... I see how it could be handy for message display as well. I use a different method, but might try this as an alternative.

Yes, Lancie, I'm still around. Just not as often. I guess being swamped with work is a good thing, but it doesn't allow me much free time to keep up with the forum.

Many thanks as always!... :site:



-Eric

As a followup, this worked out great!... :site:

I also needed a way to turn off an individual bit based on an inspection result. I used this method to determine where to perform the inspection, but also stored the decoded value in a separate register. I then XORed this with the shift register to turn off the same bit that initiated the inspection (to flag a failure). Worked perfectly!

Thanks again, Bernie!...



-Eric