Reference Manual

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PMAC 2 Software Reference

PMAC On-Line Command Specification 261

Acc 34x manual) plus 2. Any attempt to read a TWS type M-variable defined in this manner

(with bit one of its address set to 1) returns zero and the actual read is prevented by PMAC

firmware.

Because you can not directly access the individual bits of the “thumbwheel port on an

Accessory 34x board and because of the relatively long time it takes to clock the data in or

out of PMAC (A 32-bit Read or a 32-bit Write to an individual port takes approximately 64

microseconds of time in the PMAC’s background time slot) it is best to keep an “image” of

each M-variable of this type in internal memory. The image variable would preferably be a

32-bit or 48-bit fixed point M-variable, but it could also be a 48-bit floating point P or Q

variable.

The best procedure for using TWS M-variables in a program is as follows. The input word

(TWS M-variable) should be copied into its image variable at the beginning of a sequence of

operations. The operations can then be done on the image variable without requiring PMAC

to actually read or write to the I/O port for each operation. The output word is first

“assembled” into its image variable, and then copied to the actual output word once at the

end of a sequence of operations. This procedure will allow the most efficient and flexible use

of TWS M-variables.

This type of variable can only be used in background tasks (PLCs and PLCCs 1-31). They

cannot be used in foreground tasks (motion programs and PLC and PLCC 0).

Example

To address Port B of board #1 as an output using M101, use the following definition. This

addressing format is not recommended because accidental reads of the port are not protected

against (Consider using the write-only format).

M100->TWS:4 ;Port B (BIO 0-31) of an ACC-34x with SW1 switches all on

;not assigned for write-only

To address Port B as above using the write-only addressing format, use the following

definition:

M100->TWS:6 ;Port B (BIO 0-31) of an ACC-34x with SW1 switches all ON

;assigned for write-only (6=4+2)

To address Port A of board #1 as an input using M99, use the following definition. This

addressing format is not recommended because accidental writing to the port is not protected

against (Consider using the read-only format).

M99->TWS:0 ;Port A (AIO 0-31) of an ACC-34x with SW1 switches all on

; not assigned for read-only

To address Port A as above using the read-only addressing format, use the following

definition:

M100->TWS:1 ;Port A (AIO 0-31) of an ACC-34x with SW1 switches all on

;assigned for read-only (1=0+1)

Yet another example: to address Port A of board #6 as an input using M300, we would use

the following definition:

M300->TWS:41 ;Port A (AIO 0-31) of an ACC-34x with SW1 switches

; ON, ON, OFF, ON, ON assigned for read-only (41=40+1)