Multidrop / Master+Slaves on RS485 or similar

[hippyau]

Is there any possibility of using this between 2 or more nodes in a small network, with simple addressing? Or any suggestions of a protocol that is light and can support more than 2 nodes?

[lexus2k]

Hi,

Unfortunately, the full duplex version (tiny_fd) doesn't work out of the box with the RS485 physical channel, since RS485 is the half-duplex interface. Tinyproto implements only the ABM part of the HDLC protocol that can be used only with P2P connections. For your case NBM or ARM parts of HDLC must be implemented.

However, you can still write communication using tinyproto's hdlc_ll functions. The low level will provide you error checking and framing.

[michcfr]

Hello,
I got the same kind of question but with classical UART like in the figure below:
http://www.sampson-jeff.com/article/interface/as-mdd.gif
Can I address 1 master and multiple slaves?
thx

[lexus2k]

Hi @michcfr ,

According to your picture there are 2 diodes blocking TX line for 2 slave devices. ABM mode is the mode that requires confirmation, so any node in the network must have the ability both to read and send data asynchronously. Your scheme doesn't allow that, thus tiny_fd cannot be used in your case.
So the best way with the current HDLC and hardware implementation is to use hdlc_ll API and to interpret the first byte of the buffer being sent at application protocol as an address field. On the slave devices, upon receiving the whole frame, you need just to check the first byte of the buffer to understand if the data are designated for a specific slave station.

PS. HDLC NBM and ARM modes have address fields, however in many HDLC implementations the field is not used and usually initialized to either 0xFF or 0x01/0x03. The tinyproto library initializes that field to 0xFF all the time.

[michcfr]

Hello Aleksei,
Thank you for your response.

The diodes are not blocking the TX line for the 2 slave devices. Diode exists just for electrical protection reasons only.
UART uses negative logic. So, if the Tx output is high the diode is reverse biased: it blocks. If Tx output is low the diode is forward biased: it is conducting. Consequently, any node have the ability to read and send data asynchronously. So I think tiny_fd can be used.

In the case of tiny_fd, can you please show me in the source code where I can get the address fields? I suppose this is also a fixed value of 0xFF?

[hippyau]

The diodes are not blocking the TX line for the 2 slave devices. Diode exists just for electrical protection reasons only.

This is my understanding also.

In the case of tiny_fd, can you please show me in the source code where I can get the address fields? I suppose this is also a fixed value of 0xFF?

tiny_fd_int.h line 51: ​        ​uint8_t​ address;

I'd like to have simple arrangement and coordinate where the master polls the slave, and only the addressed slave responds.

[michcfr]

ok I see.
I have to do some test to see how it fits together.
For information, I plan to connect together many ESP32 throught the UART with a multi-master architecture: no master ESP32, so that any ESP32 can communicate with each other. I think this is ABM mode?
Can I add an extra field to put the source address, knowing that HDLC include only the destination address?
Another question: do you have any plan to adapt your software to Arduino ?

[lexus2k]

Hi

The diodes are not blocking the TX line for the 2 slave devices

Oh, my bad. I was confused with the diagram.  Some of the hardware manufactures require pull-up resistors for the Tx-Rx line.  Absolutely agree with you

In the case of tiny_fd, can you please show me in the source code where I can get the address fields? I suppose this is also a fixed value of 0xFF?

You can check the file tiny_fd.c. Look for .address text.

I'd like to have a simple arrangement and coordinate where the master polls the slave, and only the addressed slave responds.

Good point, but that requires a different HDLC mode - not ABM, maybe NRM.

For information, I plan to connect together many ESP32 throught the UART with a multi-master architecture: no master ESP32, so that any ESP32 can communicate with each other. I think this is ABM mode?

ABM mode is usually invoked with async modes, when each station can send at any time, and there are no master and slave devices. Current implementation of the ABM HDLC mode can be used only in P2P connections. The user data is transferred in I-Frames, which need confirmation from the receiver, and there is only a single N(s) N(r) table in the current tinyproto implementation. For your case, the master needs separate N(s) N(r) tables and different frame pools for each slave station.
I'm thinking of P2M support, but I have no solution right now for you. I'm sorry.

Can I add an extra field to put the source address, knowing that HDLC includes only the destination address?

The address field in the HDLC is exactly for the same purpose - it should contain a slave address, whenever the master sends to and receives from the packet.

Another question: do you have any plans to adapt your software to Arduino ?

Is this question for me? The library can be used in Arduino IDE as is.

[michcfr]

Hello Aleksei,
About the diodes
=> don't worry, I was confused too at first time :-)

I'd like to have a simple arrangement....
=> It was not my question, but your response is interesting

ABM mode is usually invoked with async mode....
=> yes I need P2M support where each node needs separate N(s) N(r) tables and different frame pools. Can you please indicate part of the code that need to be changed?

thx

[lexus2k]

Can you please indicate part of the code that need to be changed?

That's not so easy to update that code to work with multiple stations. I mean tiny_fd.c

[lexus2k]

Here are the steps, which are required to reach the goal.

1. Implement frame queue to keep as much frames on the master as required to support all slave.

For example, if you have window size (maximum number of unconfirmed frames) for each slave station equal to 4, then the master device must be able to keep 4 * N unconfirmed framed. If we're talking about the picture, then 4 * 2 = 8.
I'm working on this, and do not have ready solution yet.

This step also requires on the master side to have next_ns, confirm_ns, nr, last_ns counters for each slave station.
And I'm working on this.

2. Make the master to be able address each slave separately

This step required API change, because you need somehow to inform the protocol that specific data is for specific slave station only. There are 3 ways to implement that:

• Change current API (I don't like that, because it breaks the applications already using the tinyproto)
• Add additional API, something like tiny_fd_set_slave_address(handle, address) (I don't like that also, because such API can be a headache in multi-threaded applications)
• Add new API, something like int tiny_fd_send_packet(tiny_fd_handle_t handle, uint8_t address, const void *buf, int len);

3. Make the slave devices to understand, if the received frame addressed to specific section

Actually this can be easily implemented, for example, by adding the address field to initialization data for tinyproto.

4. Update the tinyproto implementation to use P/F HDLC bit correctly in NRM mode.

In current implementation P/F bit is always set to 0 for I-Frames, and is always set to 1 for S and U-Frames. Because the protocol actually fully asynchronous, i.e. each station can transmit at any time it wants. But that plays a bad role in your case, because of the collisions, when 2 slave devices want to send data at the same time.
So to solve this problem, the protocol should be significantly updated to use something like markers, and with specific timeouts, if some device doesn't respond.