Modbus Master Siemens S7-300 - OB35?

[russg]

Hi,

I'm setting up a modbus master on an S7-300. It seems to work in OB1 but not when I use it in OB35. Does anyone have any ideas why?

Could this be due to the process image selection in the CP cards hardware config?

Thanks

Russ

 

[JesperMP]

Are you using a CP341 as Modbus master ?
Are you using a sample project or library blocks ?
In the siemens document 'Master-Slave Communicationwith Modbus RTU Protocol forS7-300 and ET 200S Systems the blocks Modbus_master and S_MODB are called in OB1.
I speculate that the blocks must be called cyclically to 'service' the communication, and it is not allowed to have gaps in the calls.
Any reason to why you want to use OB35 and not OB1 ?

Could this be due to the process image selection in the CP cards hardware config?

Provide more details about the hardware setup and the program.

 

[russg]

Are you using a CP341 as Modbus master ?
Are you using a sample project or library blocks ?
In the siemens document 'Master-Slave Communicationwith Modbus RTU Protocol forS7-300 and ET 200S Systems the blocks Modbus_master and S_MODB are called in OB1.
I speculate that the blocks must be called cyclically to 'service' the communication, and it is not allowed to have gaps in the calls.
Any reason to why you want to use OB35 and not OB1 ?


Provide more details about the hardware setup and the program.

Hi Jesper,

thanks for the response.

Yes, it is a CP341 card.

Not my choice to use OB35, I'm just following specification. The explanation is because the system should only call Modbus comms every 60ms rather than every scan of OB1.

We have a function that uses P_SND_RK and R_RCV_RK to send and receive data.

in the hardware config for the CP card I only have OB1-PI an PIP 1 for the process image.

 

[JesperMP]

The explanation is because the system should only call Modbus comms every 60ms rather than every scan of OB1.

Please elaborate. Someone specified this ?

The P_SND_RK and R_RCV_RK uses a rising edge on the REQ pin to start a send or receive sequence. I can imagine that not calling the block cyclically messes that up.
Maybe there are other potential issues if the CPU doesn't cyclically update the data exchange with the CP341.

 

[JesperMP]

In the manual SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master is stated:

You will have to open the blocks P_SND_RK und P_RCV_RK several times to completely process a single MODBUS master job. The data volume you write or read determines how often you will have to open the blocks. You accelerate data exchange if you open the blocks P_SND_RK and P_RCV_RK in the OB1 cycle of the CPU. Data exchange will take longer if you open the blocks in "slow" cyclic interrupts.

So it should be possible to call in OB35.

Any code on the ERROR output pin ?

 

[JesperMP]

Did you setup the cyclic interrupt in the hardware configuration of the CPU ?

 

[russg]

Please elaborate. Someone specified this ?

The P_SND_RK and R_RCV_RK uses a rising edge on the REQ pin to start a send or receive sequence. I can imagine that not calling the block cyclically messes that up.
Maybe there are other potential issues if the CPU doesn't cyclically update the data exchange with the CP341.

Yes, I think they are worried calling the modbus comms every scan will slow down the processing of the PLC. I don't think it will be an issue, but not my choice at the moment. It may end up being that way if comms are too slow or not possible via OB35.

 

[russg]

In the manual SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master is stated:


So it should be possible to call in OB35.

Any code on the ERROR output pin ?

not on the machine at the moment. I'll let you know when I get it.

 

[russg]

Did you setup the cyclic interrupt in the hardware configuration of the CPU ?

No. How do I do that?

Did you setup the cyclic interrupt in the hardware configuration of the CPU ?

Just looking. Do I need an offset setting for it to work?

 

[JesperMP]

It should be enough to add OB35 to the program, the CPU will then call it with a cycle as set in the hardware configuration.
The 100 ms is the standard value, maybe this is too long interval for the P_SND_RK and R_RCV_RK blocks.
Offset is used avoid multiple cyclic interrupts triggering at the same time. You don't need it.
Try with 50 ms instead of 100 ms.
Check what is the return value of ERROR.

 

[Lare]

Are you using a CP341 as Modbus master ?
Are you using a sample project or library blocks ?
In the siemens document 'Master-Slave Communicationwith Modbus RTU Protocol forS7-300 and ET 200S Systems the blocks Modbus_master and S_MODB are called in OB1.
I speculate that the blocks must be called cyclically to 'service' the communication, and it is not allowed to have gaps in the calls.
Any reason to why you want to use OB35 and not OB1 ?


Provide more details about the hardware setup and the program.

On ET200S you can query modbus on intervals but you need to listen after sending message. Also you can't send new message before message before is done or timeouted.
Usually called new message after several 100 or 200ms caps between different querys. Modbus sending and listening also takes several PLC cycles.
ET200SP IM & ET200SP PtP card is more easy to use on 300 CPU than than older hardwares and there is no license either needed for modbus.