Hi,
if you send a request, a response is needed. If no response is received in parameterized monitoring time, then an error message is shown on ERROR and STATUS. The output parameters are shown only 1 plc cycle. Did you save the outputs DONE, ERROR and STATUS to check, what is shown this 1 cycle?

Could you attach the online IDB of the modbus block?

Best regards,
Wüstenräuber

Which bit you try to reset? 
You start a telegram with a positive edge to ENQ_ENR. After start you have to set the bit to FALSE. It is only a positive edge, not a positive level.
When you start a telegram with ENQ_ENR (or ENQ), then there is always a result: either DONE = TRUE, if it is successful or ERROR = TRUE, if it is not successful.

Can you attach the project? It seems that the call/handling of modbus block is not correct.

Without more infos it is not possible to find the reason. You could attach a screenshot of the OB, in which the modbus block is called. 

Which values do you parameterized for RECV_TIMEOUT and CONN_TIMEOUT?

The parameters are not correct. 
You have to delete the comments before ENQ_ENR:
A “CONTROL_DAT”.ENQ_ENR
R “CONTROL_DAT”.ENQ_ENR

And you have to set ENQ_ENR, when DONE_NDR or ERROR are set. (not reset)
O “CONTROL_DAT”.DONE_NDR
O “CONTROL_DAT”.ERROR
S “CONTROL_DAT”.ENQ_ENR

Please have a look at the manual or the block online help how the modbus block works. 
You have to set ENQ_ENR to TRUE to start 1 modbus telegram. After calling of modbus block with ENQ_ENR = TRUE you have to reset ENQ_ENR = FALSE, because the modbus block needs only a positive edge to start the telegram, not a permanent signal. 
That is why you need these lines below the call:
A “CONTROL_DAT”.ENQ_ENR
R “CONTROL_DAT”.ENQ_ENR

During the modbus block works, BUSY is TRUE. When the telegram is finished, DONE_NDR or ERROR are set for 1 PLC cycle. 
If you want to see these bits more than 1 PLC cycle you have to save it in another memory or you increase a counter. Also the STATUS output should save into another memory area, if it <> 0. 
Have a look at the original example project. In this project the handling is programmed.

If you want to start the next modbus telegram, you have to set ENQ_ENR = TRUE:
O “CONTROL_DAT”.DONE_NDR
O “CONTROL_DAT”.ERROR
S “CONTROL_DAT”.ENQ_ENR
But you may only set it to TRUE, after the previous telegram was finished.
 

You can also attach the online IDB of the modbus block. Then I can have a look at the internal parameters.

Hi,
change the parameter for “connect_startup” to FALSE in parameter DB and re-initiate the modbus block. I know, that sometimes it is a problem with communication partner, if this bit is set to TRUE at start. 
(If the connection is not established, when ENQ_ENR is set to TRUE, then the connection establishment is initiated automatically. That is why it is not a problem to set this parameter to FALSE.)

Wüstenräuber

Attach a new picture of the modbus block call in OB1 and the onlineIDB as pdf. 

When you open the OnlineIDB of the modbus block, you can print the IDB as pdf. It is more easierer than screenshots. ;-)