The FB you created, does it have any references to any global M or DB tags inside the block?

You want all required parameters passed into and out of the FB.

(Not applicable in your case, but others may read this later...) If you are using an FC, you will not succeed (without losing your mind first).

If it works with a single remote MB device at a time, you need to ensure you are not using the same values for things.

I am assuming both remote MB devices are networked together over RS485.  If that assumption is correct, you only have a single serial comms modules, correct?

If my assumption is incorrect and you have 2 distinctly different cables going from the PLC comm module(s) to 2 remote MB devices, the correct hardware ID needs passed into the FB for the right remote MB device.

If they are networked (most likely), you need to keep track of the BUSY signal from the MB send instruction calls, and pass that out.

In the main PLC code, you need to manage that busy signal and wait for it to complete before sending another message.

By the way, when you looked up the 16#8387 error output, what did it say in the error description?

AutoGen_2379672

Hello everbody,

I created a Fb. I am using In Fb modbus master and modbus_comm_load.

There are 2 identical devices communicating via modbus rtu.

Device 1 : 9600,8,none,1 mb adress:1

Device 2:  9600,8,none,1 mb adress:2

The cpu I use is s7 1500 and PTP remote communication module with IM module. The block I created communicates with devices individually. However, when I call the block a second time and want them to communicate at the same time, the mb_master in one of the blocks gives the error 16#8387. What is the reason for this?

Thanks.

From a modbus network point of view, it is fine asking multiple devices for data at the same time.  However, you only have 1 comm module communicating via RS485.

There are multiple ways to approach it, the easy way and the hard way.  Easy way is to use the provided MB_... blocks to send requests.  This is the preferred way and you can use the F1-Help for the block you are probably already using.

The hard way - to essentially built your own version of the block around the basic PTP communication send / receive blocks.  This method allows sending to device 1, then device 2, and listening on a single PTP receive call and then analyzing the returned results which includes the MB address in the returned packet.  Lot more work, but it does allow requesting and receiving from multiple devices much faster than the easy way.

The easy way sets up everything, sends the request to a single device, waits for the response and provides the returned data in a simple and predicable manner with very little work.  It does require the PLC to sit and wait for the first request to complete before attempting to request from the 2nd device.

Have you downloaded any sample applications where Siemens shows how to request from multiple MB devices using their MB blocks?

Perhaps you can try searching this support site.

For example, I found this prior forum topic that may be helpful:

https://support.industry.siemens.com/forum/WW/en/posts/how-to-use-multiple-modbus-master-db-s-in-one-program-with-the-mb-comm-load-db/269577

From documentation for the MB_MASTER block:

Modbus master communication rules

• MB_COMM_LOAD must be executed to configure a port before a MB_MASTER instruction can communicate with that port.
• If a port is to be used to initiate Modbus master requests, that port should not be used by MB_SLAVE. One or more instances of MB_MASTER execution can be used with that port, but all MB_MASTER execution must use the same MB_MASTER instance DB for that port.
• The Modbus instructions do not use communication interrupt events to control the communication process. Your program must poll the MB_MASTER instruction for transmit and receive complete conditions.
• It is recommended that you call all MB_MASTER execution for a given port from a program cycle OB. Modbus master instructions may execute in only one of the program cycle or cyclic/time delay execution levels. They must not execute in both execution priority levels. Pre-emption of a Modbus Master instruction by another Modbus master instruction in a higher priority execution priority level will result in improper operation. Modbus master instructions must not execute in the startup, diagnostic or time error execution priority levels.
• Once a master instruction initiates a transmission, this instance must be continually executed with the EN input enabled until a DONE=1 state or ERROR=1 state is returned. A particular MB_MASTER instance is considered active until one of these two events occurs. While the original instance is active, any call to any other instance with the REQ input enabled will result in an error. If the continuous execution of the original instance stops, the request state remains active for a period of time specified by the static variable Blocked_Proc_Timeout. Once this period of time expires, the next master instruction called with an enabled REQ input will become the active instance. This prevents a single Modbus master instance from monopolizing or locking access to a port. If the original active instance is not enabled within the period of time specified by the static variable "Blocked_Proc_Timeout", then the next execution by this instance (with REQ not set) will clear the active state. If (REQ is set), then this execution initiates a new master request as if no other instance was active.

If you are following all those rules, everything should work - provided you also followed the requirements for the various block parameters.

It's rather tricky how they require the re-use of the instance DB for all MB_MASTER calls.

This forum topic has a link to an example application plus other suggestions for PLC code implementation concepts: https://support.industry.siemens.com/forum/WW/en/posts/s7-1200-modbus-rtu-with-12-slave/178837