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Entry type: FAQ Entry ID: 21562130, Entry date: 08/07/2006
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What must you watch out for when connecting PROFIBUS PA devices to the PROFIBUS DP?

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Configuration notes:
These devices are connected to the PROFIBUS DP directly with a DP/PA coupler or via a DP/PA link.
The transfer rate on the PROFIBUS PA is 31.25 kbaud in both cases.

In the case of the direct connection to the PROFIBUS DP with the DP/PA coupler the data transfer rate on the PROFIBUS DP is limited to 45.45 kbit/s. The DP/PA coupler converts the messages from the PROFIBUS DP simply into PROFIBUS PA messages and also produces the operating current for the PA slaves.

In the case of connection via a DP/PA link you can select a data transfer rate on the PROFIBUS of up to 12 Mbit/s. The DP/PA link is a DP slave on the PROFIBUS DP (primary side) and the PA master on the PROFIBUS PA (secondary side). The DP/PA link exchanges data with the PA slaves on the secondary side via the DP/PA coupler.

Options for transfer from PROFIBUS DP to PROFIBUS PA:


Fig. 01

Number of devices that can be connected:
You can connect 125 PA slaves to a DP master system distributed over multiple DP/PA couplers.

If you are using DP/PA links with a DP/PA coupler connected, then up to 31 or 64 PA slaves can be connected distributed over multiple DP/PA couplers. The number of PA slaves depends on the version of the DP/PA link:
 

MLFB of the DP/PA link Max. number of PA slaves
6ES7 157-0AA00-0XA0 31
6ES7 157-0AA80-0XA0 31
6ES7 157-0AA81-0XA0 31
6ES7 157-0AA82-0XA0 64

If you want to connect more devices, then you must use another DP/PA link with DP/PA coupler.

Configuration and parameterization message
The maximum length of the messages for configuration and parameter data on the PROFIBUS DP is 244 bytes per DP slave. Consequently, when configuring via GSD file only 244 bytes of configuration and parameter data can be assigned per DP/PA link. In certain circumstances this can lead to a reduction in the number of field devices connected per DP/PA link.
When configuring the DP/PA link from the meta-knowledge of STEP 7 (in the catalog under PROFIBUS DP > DP/PA link > IM 157), STEP 7 checks the device limitation and does not permit incorrect configurations.

Slot restrictions with the DP/PA link
When configuring the DP/PA link via GSD file, one PA slave might occupy several slots. The maximum number of slots of a DP/PA link is 236.

I/O data area:
The entire address space of all the field devices connected to a DP/PA link can be up to 244 bytes. In certain circumstances this can lead to a reduction in the number of field devices per DP/PA link.

If you want to connect more devices, then you must use another DP/PA link with DP/PA coupler.

Power consumption:
If the field devices are not in hazardous areas, then the power consumption of all the field devices connected must not exceed 400 mA per DP/PA coupler 6ES7 157-0AC80-0XA0 or 6ES7 157-0AC81-0XA0, or 1000 ma per DP/PA coupler 6ES7 157-0AC82-0XA0. 
If the devices of a DP/PA coupler Ex [i] (6ES7 157-0AD81-0XA0, 6ES7 157-0AD82-0XA0) are used in hazardous areas, then the total power consumption must not exceed 110 mA. In most cases therefore the number of field devices that can be connected is limited to about 10 per DP/PA coupler Ex [i].

If you want to connect more devices, then you must use another DP/PA coupler (max. 5 per DP/PA link).

Summary:
You can use the following formula for the theoretical maximum number of PA slaves per PROFIBUS master system (max. 125 DP slaves per master system):

  • Ex version (hazardous area) of the DP/PA link (6ES7 157-0AD82-0XA0):
    Number = 125 DP/PA links * 5 DP/PA couplers * 10 PA slaves (per DP/PA coupler)
    = 6250 PA slaves.
  • Non-Ex version of the DP/PA link (6ES7 157-0AD82-0XA0):
    Number = 125 DP/PA links * 64 PA slaves per IM157
    = 8000 PA slaves.

Notes:

  • Direct connection of the DP/PA coupler to the PROFIBUS DP:
    The DP/PA coupler is not configured explicitly in the HW Config. You configure your PA devices like DP slaves. Addressing is direct from the programmable controller.
    The bus addresses of the PA slaves after the DP/PA coupler must not overlap with the those of the nodes of the associated DP master system, because these field devices are considered as being connected to the DP bus with regard to the addressing.
  • Connection of a DP/PA link to the PROFIBUS
    The DP/PA link is a slave on the PROFIBUS DP and a master on the PROFIBUS PA. Two bus systems linked by the DP/PA link are created in HW Config. The programmable controller/automation system addresses the field devices via the DP/PA link like a modular slave whose modules are the PA devices.
    The bus addresses of the PA slaves must be unique after a DP/PA link, it is not enough to specify unique addresses after each coupler of a DP/PA link. The addresses in the PA master system are specified independent of the DP master system to which it is connected. Address range of the PA slaves: 3 ... 124.

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