14
Operation
Acyclic Operation
The descriptions above refer to cyclic, i.e. continuously repeated
data transfer. If, however, values like control parameters or
setpoints have to be written sporadically for these devices, the
cyclic system can no longer be used. - Therefore, PROFIBUS-DP
offers for these devices an interface for acyclic operation.
This kind of operation permits both acyclic writing and acyclic
reading of data. -
First of all you have to find out if the used PROFIBUS-DP master
already supports the standardized services of Version 1 called
PROFIBUS-DP V1.-- Otherwise, you can also use the realization
without PROFIBUS-DP V1 which is described later in this manual.
Realization with PROFIBUS-DP V1
The standardized version of PROFIBUS-DP V1 provides 2
services for acyclic communication, called READ (for acyclic
reading) and WRITE (for acyclic writing). These services are
available for PROFIBUS-DP masters of Class 1 and Class 2.
Data addressing for communication is mainly based on two
description values:
Slot
and
index
. Every pollable controller value,
variable or online parameter can be addressed in a unique way
through slot and index.
Refer to section "Tables" for realizations without PROFIBUS-DP
V1 that are already in use. You can find there an ID for every
addressable value. - This 16-bit index is also called a variable
index. The addresses for PROFIBUS-DP V1 services are based
on this variable index. For this purpose, the 16-bit variable index
(read hexadecimal) is split up in its high and low part. For DP V1
addressing the high part is used as slot, and the low part as
index. .
Example
Variable .L3_R1
16-bit variable index
0x023B(hexadecimal format)
Slot
0x02
(hexadecimal format)
Index
0x3B
(hexadecimal format)
This method is also applicable for the used variable indices of
online parameters, for which the variable index can be calculated
on the basis of special information (see section "Addressing the
Data"). -
When using this variable index, take care not to use the values
255 or -0xFF, as they are predefined values. Since there are
values within the existing variable indices which may assume the
value 255 or 0xFF, these values must be changed. No problem
will arise if slot is set to one of these values (255 or 0xFF). If
however, a value of 0xFF results for index, use value 0x00 for
index and increase slot by 0x80.
Example
Variable .L3_T1_STEUER
16-bit variable index
0x01FF(hexadecimal format)
Slot
0x81
(0x01 + 0x80; hexadecimal format)
Index
0x00
(0xFF
→
0x00; hexadecimal
format)
