Skip to Content

SiePortal

The integrated platform for your product selection, buying and support workflow - bringing together Industry Mall and Online Support.

SiePortalDeutsch

Search

Home
Support Support
{{(apiData.Support | filter:{'rank':1}:true)[0].title}} Support
- Menu temporary not available
- Menu temporary not available
- Menue temporary not available
- {{item.title}}

Products & Services
- Menu temporary not available
- Menue temporary not available
- {{item.title}}
Support Support
{{(apiData.Support | filter:{'rank':1}:true)[0].title}} Support
- Menu temporary not available
- Menue temporary not available
- {{item.title}}
MySiePortal MySiePortal
{{(apiData.MySiePortal | filter:{'rank':1}:true)[0].title}} MySiePortal
- Menu temporary not available
- Menue temporary not available
- {{item.title}}
Cart ({{apiData.CartCount}})

Navigation

Theoretical information about Modbus Network

Created by: r2407 at: 12/6/2010 12:25 PM (14 Replies)

Rating (0)

Thanks 1

Result pages:: |; 1 | 2; |

15 Entries

Entries per page: 10 | all

12/9/2010 2:44 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 4/19/2025 Posts: 12346 Rating: (2710)	Hello r240`; When you use a Siemens Cp as a Modbus master,you use the Send and recieve blocks with a dediacted DB for each Modbus call. This DB must contain the Modbus address of the slave, the Modbus Function used, the start address in the slave of the accessed data, and the length of data required. Please study the Modbus Functions chapter and the sample program shown in the CP 341 Modbus Master manual to understand this mechanism. To access data from one slave, you must use one set of Send/Recieve blocks with the dedicated DB. For every subsequentcall of a Modbus function, you must reuse the Send/Recieve blocks each time, with a dedicated DB every time. So if you have 10 slkaves, and you are accessing 2 areas of data in each, you will need 20 Modbus Senbd/Recieve blocks, and 20 distinct DBs, just to manage the Modbus jobs. Yoiu cannot call more than one pair of Send/Recieve blocks at a time. Spo you must call them one at a time, or sequence them. You can either use a timed interval to trigger each call, which will make your Modbus cycle very slow, or you can use the interface bits (DONE in FC 41 and NDR in FC 23, in the sample program) to make sure the Send/Recieve blocks are finished for one Modbus call befpore you call the next one. hope this helps, Daniel Chartier

Suggestion To thank Quote Answer

12/9/2010 4:23 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello, I do not fully agree with dchartier. I recommend to call P_SND_RK only once in the way shown below (DB42 has to be filled in regarding the needed requests). I modified the block of the example project a ittle bit. Regards, Kaulquappe // ------------------------ // SEND with Instance-DB // ------------------------ CALL "P_SND_RK" , "IDB_P_SND_RK" SF :='S' REQ :=DB40.DBX0.0 R :=DB40.DBX0.1 LADDR :=DB40.DBW16 DB_NO :=#request_db DBB_NO :=#request_dw LEN :=#request_len R_CPU_NO:= R_TYP :='X' R_NO := R_OFFSET:= R_CF_BYT:= R_CF_BIT:= DONE :=DB40.DBX0.4 ERROR :=DB40.DBX0.5 STATUS :=DB40.DBW12 U DB40.DBX 0.0 //SEND_REQ R DB40.DBX 0.0 //SEND with REQ=0 // ------------------------------- // Check "Complete without error" // ------------------------------- U DB40.DBX 0.4 //DONE ? SPBN CON1 //if NO L DB40.DBW 6 //"Complete without Error" + 1 //increment counter T DB40.DBW 6 L #sequence SPL XXX SPA seq1 SPA seq2 SEQ1: L 42 T #request_db L 0 T #request_dw L 6 T #request_len L #sequence INC 1 T #sequence SET = DB40.DBX 0.0 SPA leav SEQ2: L 42 T #request_db L 6 T #request_dw L 6 T #request_len L #sequence INC 1 T #sequence SET = DB40.DBX 0.0 SPA LEAV XXX: L 0 T #sequence SPA LEAV
Kaulquappe Posts: 3093 Rating: (324)	Last edited by: Kaulquappe at: 12/14/2010 3:25 PM
Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users J_Bell

12/11/2010 7:42 AM	Rate (0)
r2407 Posts: 111 Rating: (0)	Hi dchartier, I have copied the prepard program existing in MB_ASCII of sample prject to my program. It contains OB1, OB100, FB7, FB8, FB10, FB21, FB23, DB40, DB41, DB42, DB43, DB50, DB70 and VAT1. I want youexplain me which one is send and receive blocks and which DB is its dedicatd DB. After putting slave address, the modbus function used, the start address and the length of data at the proper DB how could I access to these addresses and how to move to other slave.? How can I start sending data autmaticallybecause there is aM120.7 within the program ? and finally where were these read valued stored? Regards
r2407 Posts: 111 Rating: (0)
Suggestion To thank Quote Answer

12/14/2010 10:39 AM	Rate (0)
r2407 Posts: 111 Rating: (0)	Dear all experts, Is there anyone to help me, I have really gotten cnfused by a great deal of documents.
r2407 Posts: 111 Rating: (0)
Suggestion To thank Quote Answer

12/14/2010 3:37 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello r2407, for each read request the call of FB P_RCV_RK is necessary. I recommend the call of FB P_RCV_RK once with a static data destination. When FB P_SND_RK sets the DONE bit, the received data are already entered in the receive DB. Then the received data should be copied to the data destination of the responding slave. How the data are entered in the receive DB is shown in section 5 of the Modbus master manual. The function of FC21 and FC22 is described in section 8.1.2 of the manual. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Follow us on