Hi.

(TL;DR below)

I have re-written our modbus code several times after trying several ways. There is always a problem with MB_CLIENT.

First we have a local variable with TCON_IP_v4 structure, where we set ip and port, so far so good.
Then we have _MB_CLIENT as a local struct of MB_CLIENT type.

Currently we have a step-by-step function that for example #step=1, we put data in the _MB_CLIENT
like:

.MB_MODE = 0
.MB_DATA_LEN = 2
.MB_ADDR = 42000
.MB_Unit_ID = 1 

This translates to we want to read 2 registers in the register 2000 (and 2001) from modbus unit 1.
Using address 32000 or 42000 makes no difference in what i can see, i get the same data or nonsense data.

Now i have an array 1..10 registers i want to read out one by one, so i have this #RegisterIdx local variable that i increment in the end after i get _MB_CLIENT.MB_ERROR or _MB_CLIENT.MB_DONE.

I have been reading up alot today, and i get very conflicting information on how to use the MB_CLIENT block.

One place says i should only hold the input REQ high until i get MB_DONE, another says i should always have input REQ high as long its connected.
Another says i should have input REQ high only for 100ms.
I have tried all three ways. When the REQ is not high until done/error, the request never seems to finish, and mb_client seems to stall.
If i keep the REQ high until done/error comes, i sometimes see varying error codes such as

error codes

818D
The transaction ID (MB_TRANSACTION_ID tag) does not correspond to the one sent originally (see static tags of instruction).

8200
A different Modbus request is currently being processed via the port.
Another instance of MB_CLIENT with the same connection parameters is processing an existing Modbus request.

8383

Error reading or writing data or access outside the address area of MB_DATA_PTR.
The error can occur locally as well as with the instruction "MB_SERVER".

 

The modbus device we are trying to communicate with is a Schneider-Electric PowerLogic EGX100 / Compact NS and Masterpact NT/NW, these units meassure power consumption of varying devices in the cabin, like welding robots or conveyers.

I like to belive that i now grasp how to communicate via the MB_CLIENT, but it only seems to work on some occations.

Not to make the programming too complex, i have tried to only make a look 1-10 and ONLY communicating with 1 MB unit (instead of looping thru 1-10 for each 1-10 units).
The first few runs with the communication (atm now 2), all starts to get more and more errors, where i should expect to get 4 MB_DONE with good replies, i only get 2, and then i get several MB_ERRORs (counting then on a positive edge).

The plc on my desk has now been running for 60 minutes, i have 2 good reads, and almost 600 errors

1. In short how the loop are:
2. Read loop index and get data from an array in a datablock, and populate such things as mode, len and addr.
   
3. Put the input legs of mode, addr and length + pointer array 0..9 of words.
4. Enable request
5. Wait for done/error, lower request
6. Read pointer data and move it to another array to be processed later.
7. Clear/reset various variables to initial state, like an empty read pointer, flags set to false etc
8. Loop index incremented by 1
9. Restart the loop (with the new register i want to read from the modbus device)

--

Most help i find, is for s7-1200, which does not work as in s7-1500.
Another problem i get often, is when i download new changes, and there is a communication during that time, i always get mb_error and error code80A3 until i put the cpu in stop and then restart it.

Somewhere i also read that using a local variable for the output of done/error etc, and then uses that local variable, would trigger something to revaluate the instance db for mb_client of the mb_done/mb_error.

--

LT;DR:

How is the s7-1500 mb_client be handled when it comes to trigger REQ, populate data, using local variables for done/error/busy signals (outputs of mb_client).
I will need to read from 1..n devices on the same ip for the mb_client (usually 1-3 units is common here).
We want 1 single block that loops thru all registers we want to read from, one by one, for each unit.
Should the code handle connection id, and transaction id, or do the mb_client handle this by it self automatically?

 Hi.

I do something similar to this, does not matter if i use busy to drop the request flag or not.
If i specify 1 unit with 4 registers, then the code loops thru without any error unless i disconnect the unit.

If i specify 2 units with the same 4 registers, and only have 1 unit connected.
Then i seem to get status error 8200/818D quite often, this mean it got a reply with a different transaction id or some other mb_client is talking on this ip+port, and then i rarely get an "ok" message (done), and most tries get error, even when i try communicate with the unit that is connected.

Kaulquappe

Hello FuriousRage,
I don't know the internal sequence of MB_CLIENT for sure, but I'm thinking about the timing of resetting REQ you already use. I would reset REQ right after the first execution of MB_CLIENT.

1. set REQ
2. call MB_CLIENT
3. reset REQ
4. wait for DONE / ERROR
5. set the values for the next job

Regards,
Kaulquappe

I have found out that modbus can read several Scattered Holding registers.
Anyone know how i can accomplish this with S7-1500 MB_CLIENT if its even are possible?
We currently want to read 4-5 different registers, and we could read out these 4-5 at once, it would solve much of our problems with the modbus.

FuriousRage

I have found out that modbus can read several Scattered Holding registers.
Anyone know how i can accomplish this with S7-1500 MB_CLIENT if its even are possible?
We currently want to read 4-5 different registers, and we could read out these 4-5 at once, it would solve much of our problems with the modbus.

 I dont think you can, but you can acomplish what you need in different callings to MB_CLIENT, so what you need to do is as follows:

1) Call first MB_CLIENT instance with the first set of registers you want to read.

2) When MB_CLIENT.DONE is 1, transfer this data to DB.

3) Call second MB_CLIENT instance with the second set of registers you want to read.

note1: you must disable instance #1 in order to enable instance#2, otherwise it will overwrite the second register set and will go to error.

note2: you must create a UDT with the fields you need to add in "PTR" field 

i've done this on a ION meter via modbus RTU. 

Sample program:

https://www.dropbox.com/s/qrlp6sh98xf4fgb/MODBUS_S71500_V13SP2.rar?dl=0

Hi everyone.

Just for the records, I'm doing it by sequencing requests of different MB_CLIENTS  on a FC and for each one a certain number of registers is passed from the server to the PLC.

1. Enable REQ bit ( only executed if NOT BUSY and no other REQ for the same  server is on)

2. Wait for DONE, ERROR or a time limit set by me with a TON instruction.

3. Reset REQ and set DISCONNECT.

4. Enable the next MB_CLIENT block REQ and so on.

Hope it helps and if you have a suggestions please let me know!.

Regards