danw

You need to be aware that it's up to you to create and implement the entire comm protocol.  It could be a challenge, but yes, the devices can be addressed and the master controls the bus communications with its command queries and waiting for the reply.

The device addresses must set via the comm link through an address command. Presumably the devices remember their addresses after the addresses are established, but the manual doesn't actually say that, so maybe the devices forget their address after they lose power ? ? ?

Presumably, after the addresses are established, the master sends two commands; first a trigger command (to which there is no reply), the master then waits at least 30mS then sends a request for the distance data.

It isn't clear to me what the SUM value is, it could be a check sum, but it isn't clear in a  brief review and I didn't do the binary arithmetic.

Header   Address Length Cmd   SetAddress   SUM

55 aa         AB        1       55        ADD          SUM

message in hex:

0x55 0xaa 0x11    0x01   0x55      0x12           0x79 (Set Address to 0x12)

ADD is the device address, some value between 0x11 and 0x80.

115K baud requires terminating resistors and can drive 30 devices over 1200m of cabling (twisted pair, low capacitance comm cable).  

RS-485 is a differential technology: The A driver line with respect to signal ground minus the the B driver with respect to signal ground.  

This device does not have a 3rd wire 'signal ground'.  So the signal ground is likely the power supply (p/s) GND (-).   As long as GND terminals are all at the same potential, then the 485 bus will be happy.  But if there is a potential difference (common mode) between the p/s GND terminals in excess of about +12 to -7 volts, the 485 bus will saturate and fail.  Using an RS-485 repeater/Isolator will generally remove the common mode and get the network up and running.

 Hi

In terms of programming which block should we use in TIA Portal can we still use send_PtP and rcv_PtP. In a way that we send command to first device wait to get back the response and then send another command to second device and we update the address in the command we are going to send and so on. Is it possible?

------------------------------------------------------------------------------------------
Split from Multiple devices with the RS485 interface communicating with an S7-1200 CPU 1215C PLC.

rezgd

Hi

In terms of programming which block should we use in TIA Portal can we still use send_PtP and rcv_PtP. In a way that we send command to first device wait to get back the response and then send another command to second device and we update the address in the command we are going to send and so on. Is it possible?

Hello.

Yes you can use those instructions to send out character string on the RS485 channel.

But understanding the protocol (read: communication rules) used by the field device is important. Standard instructions are available for the case where Modbus RTU is specified. 

What is yours specified as?

~

qwazee

 

rezgd

Hi

In terms of programming which block should we use in TIA Portal can we still use send_PtP and rcv_PtP. In a way that we send command to first device wait to get back the response and then send another command to second device and we update the address in the command we are going to send and so on. Is it possible?

Hello.

Yes you can use those instructions to send out character string on the RS485 channel.

But understanding the protocol (read: communication rules) used by the field device is important. Standard instructions are available for the case where Modbus RTU is specified. 

What is yours specified as?

~

 Thanks for your response,

in a project we have 4 pressure sensors and in the manual it was mentioned that for multipoint connection we need to make a full duplex 4W RS485 network. as you know CM 422/485 HF only supports RS485 2W, I had a discussion with the sensor provider and they confirmed we can use either RS422 4W or RS485 2W (using 4W <-> 2W converter or just changing the wiring).

on the programming side what manual says is we need to assign addresses from 1 to 31 to each sensor and send ascii command to read pressure value and the sensor will send back the response and it would be same for all other sensors with different addresses not to mention that it is not Modbus RTU we need to implement the protocol. So for the programming I want to use freeport but in the siemens manual it seems it is only for point to point connection. my question is how we have multipoint connection in RS485 2W network if we tend to implement the protocol by ourselves?

You mentioned we can still use Send_PtP and RCV_PtP can you explain the procedure a bit more in details?

How should we assign addresses to each sensor with that? do we need to connect them individually to assign their addresses and then connect all four sensor and send appropriate command with addresses to each of them?

This is a topic that has not been discussed previously as I searched a lot and read all related topics.

Hello.

You are asking specific solutions without being specific yourself. We are blind to what you are using. The documentation of the field devices should guide you in what you require.

The field devices needs to be configured before the PLC is connected. It may have a connection station, serial connection to the PC, physical selection switches or onboard LCD & buttons. A serial network of several devices does not make sense before such configuration is complete. The user manual should give guidance.

"Point to point" means you are linking one point to another point. Having multiple devices between the points does not matter. A network of devices is still valid. Just take care of the specifications of RS485 when it comes to the count of devices (32) and about terminating on both sides of the "trunk".

Again the manual of the device should guide on how the protocol should be. Some random example would be something like:

<01rd>

There are 7 characters transmitted using the SEND_PTP instruction. The `<` starts the command while `>` ends it. The `01` designate which address device needs to answer. `rd` is the short code for the PLC wanting to get the actual reading.

The addressed device would then transmit ASCII characters that can be absorbed by the PLC by the RCV_PTP.

Notice that the hardware configuration of the card has some configurations that may help to automate some processes. The system manual of the PLC is very helpful on getting an idea what each does. There is also an instruction which you can change those settings per user program. 

Though I have no 1st hand experience in using the instructions but you should find that my notes are helpful. The manual of the device is the most important starting point.

Good luck.

Thank you so much 

your explanation was very helpful and again thanks for that. as you mentioned I will first start to assign addresses to each one of sensors then I stablish the network. one thing that I wanted to make sure was the ability of Send_PtP and RCV_PtP to have a PtP communication with each one of networked devices on a bus I searched a lot but no where I could find any information about it but with your explanation I got the point. I will implement the project and I will share the results here. Just one last question our sensors support RS422 asw well if we want to have a full duplex network and want to use RS422 option still can we use those FBs and having PtP communication with each one of networked devices?

rezgd

Thank you so much 

your explanation was very helpful and again thanks for that. as you mentioned I will first start to assign addresses to each one of sensors then I stablish the network. one thing that I wanted to make sure was the ability of Send_PtP and RCV_PtP to have a PtP communication with each one of networked devices on a bus I searched a lot but no where I could find any information about it but with your explanation I got the point. I will implement the project and I will share the results here. Just one last question our sensors support RS422 asw well if we want to have a full duplex network and want to use RS422 option still can we use those FBs and having PtP communication with each one of networked devices?

The connection for 2 wire RS485 may look something like the picture:
 

Note that each message is broadcasted throughout the network. Every device hears every message. That is why there are one master and all slaves have addresses. The slave responds only when spoken to. The slave does not need to say to whom it responds to, because it only responds to the master's queries.

That is the baseline principle that many systems uses, but other methods are available in devices. It is high bets your instruments want a one master, 1 or more slaves environment.

The PLC hardware that allows RS485 and RS422 on the same card has a selection on the hardware configuration as to which method is used. The configuration instruction also have these options. That is explained in the section of the S7-1200 system manual that documents those instructions. You can only select to use one connection type.

The send and receive instructions look the same for RS232 / RS485 / RS422. Those instructions are there to send and receive ASCII characters. The hardware identity number referenced in the instruction associates the task with a physical connection. Best to read the detail in the S7-1200 manual. It is explained very completely.

~