I am unfortunately the heir to three old Modicon 984s. I need to modify the programs to perform some initialization routines on startup. I have not been able to determine if the 984 has a first scan bit or instruction that is true only during the first scan of the program. Is there such a device in the 984s, or will I have to construct one in ladder?

 

I never was aware of a first scan bit. I always created my own.

984s aren't so bad. If you received Micro 84s I might feel sorry for you

Jim

 

On the first network in the first segment place a coil against the rail. Use the rising edge of this coil to do your initialization

Rob E.

 

put ON a coil w/o any conditions, on the last line of your ladder.
use the NC contact of this coil address, for the initialisation routine

this should solve your problem.

 

I never was aware of a first scan bit. I always created my own.

984s aren't so bad. If you received Micro 84s I might feel sorry for you

Jim

 

Hello T. Connolly,

Modicon 984 PLCs do contain a first scan bit. However, you have to use the STAT function block to get the processor status information that contains the first scan bit.

The first scan bit is in the third word returned from the STAT function. The first scan bit is bit number 1 in the third word. Remember, Modicon numbers thier bits from 1 to 16 and left to right.

You can use the SENS function block to determine if the first bit is ON. The middle output from the SENS function block can then be attached to a coil. This coil can then become the first scan bit used everywhere else in the PLC. The first scan bit and the first scan coil will only be ON during the first scan of the PLC.
___________________
--| | | |
| 40501 | |#0001 |
| | | | first scan
| STAT | | | coil
| #0003 | |40503 |------------( )---
--------- | | 01001
| |
| SENS |
|#0001 |
--------

Please feel free to contact me if you have additional questions.

Bradley G. Hite
Intertech Incorporated
mailto:[email protected]
http://www.myplc.comTeaching Practical Skills for a Technological World

 

T. Connolly wrote:
>I have not been able to determine if the 984 has a first
>scan bit or instruction that is true only during the
>first scan of the program.

See the STAT instruction, Word 3 (General Controller Status), Bit 1.


Jay Kirsch