Siemens SIMATIC S7-1200 System Manual page 1200

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Communication processor and Modbus TCP

13.5 Modbus communication

● It is recommended that you call all Modbus_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 Modbus_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 Modbus_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

Modbus_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 Modbus_Master request as if no other instance was active.

REQ parameter

0 = No request; 1 = Request to transmit data to Modbus Slave

You may control this input either through the use of a level or edge triggered contact.

Whenever this input is enabled, a state machine is started to ensure that no other

Modbus_Master using the same instance DB is allowed to issue a request, until the current

request is completed. All other input states are captured and held internally for the current

request, until the response is received or an error detected.

If the same instance of Modbus_Master is executed again with REQ input = 1 before the

completion of the current request, then no subsequent transmissions are made. However,

when the request is completed, a new request is issued whenever a Modbus_Master is

executed again with REQ input = 1.

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK