Modbus RTU protocol over "BLE UART"

[Demetrio Magrin REEL]

I used the Nordic EVAL model nRF52-DK which uses the "nRF52832" chip.
The EVAL has been programmed as "Pheripheral" with the application "BLE over UART" made available by Nordic itself.
The "BLE over UART" profile contains the SoftDevice inside: "s132_nrf52_6.1.0_softdevice.hex".
The RX / TX signals of the UART have been connected to the serial port of an external device.
The BLE client is a PC application interfaced with the VCP of the "nRF51-Dongle" dongle.
The data frames to be exchanged have a maximum size of 512 bytes (stardard modbus RTU). The ATT_MTU = 20.
What is the reliability of BLE communication in the case of buffer fragmentation in packets of 20 bytes each ?
Is there a guarantee that the packets are always sent sequentially without the loss of packets ?
Which recommended solution to use for data exchange with such large buffers with modbus RTU ?

Best regards.
Thank you.

Demetrio Magrin

[Kenneth]

Hi,

The link layer of BLE will ensure all packets are successfully received by peer and in the correct order. It's only a link loss that may interfere with this.

To transmit large buffers it may be a good idea to use data length extension (include gatt_init in your project to handle this), this allow the two peers to exchange the maximum data packet size on-air, which overall will increase throughput and reduce latency.

You can in the application have a large UART buffer, e.g. 1kByte, and in addition use hardware flow control on UART. This should ensure that the application can buffer the next packet while the current is transmitted. Enabling flow control will ensure that the peer don't overrun the buffer.

Best regards,
Kenneth

[Demetrio Magrin REEL]

Thank you.
I have seen that this topic has already been dealt with at the following link:

devzone.nordicsemi.com/.../regarding-rs-485-to-ble-links-for-Modbus

In particular I saw that there is already a Texas gateway visible at the link:

www.ti.com/.../TIDC-BLUETOOTH-SMART-TO-RS-485-GATEWAY

Is there already something similar already done by Nordic ?

Thanks again for the support.

Best regards,

Demetrio Magrin

[Demetrio Magrin REEL]

Hello Mr. Kenneth,

With the examples provided by Nordic as UART over BLE (pheriferal and client) the transmission of a buffer of 300 bytes (for example), is received in 15 packets of 20 bytes with a pause between a packet and the next.
If I send, through the UART, these 15 packets to my external control I would have the intervention of the end-of-frame modbus RTU interrupt at the end of each pack of 20 bytes.
To solve this problem it is necessary to receive all the data in 20 bytes packets and then transmit all the received buffer to my control. Similar to Bluetooth BR/EDR.
Note: for iOS systems it is advisable to use an ATT_MTU of 20 bytes.
For this reason we have chosen to use ATT_MTU = 20 (default).
We ignored the possibility of using an ATT_MTU> 20 bytes.
I await a kind reply.

Best regards,

Demetrio Magrin

[Kenneth]

The UART service may be by default be setup for 20bytes, but longer is possible by modify the software. I could not find any specific requirement in iOS that prevent longer packet, they seem to support up to 251 bytes (ch. 11.11 and 11.7):

https://developer.apple.com/accessories/Accessory-Design-Guidelines.pdf 

In terms of software deliveries I guess you can find what we have to offer in the SDK documentation, maybe this link is a good starting point:

https://www.nordicsemi.com/DocLib/Content/SDK_Doc/nRF5_SDK/v15-2-0/transport_libraries 

[Demetrio Magrin REEL]

Hello Mr. Kenneth,

It confirms that to transmit BLE packages of size greater than 20 bytes I have to modify the row
#define BLE_GATT_ATT_MTU_DEFAULT 23 and then recompile my application and the related softdevice ?
This must be done for both BLE (pheripheral and client).

Is this the only way to use for MODBUS RTU over BLE communication or are there other possibilities?

Thank you for your availability.

BR

Demetrio Magrin

[Kenneth]

Hi,

There is really no short answer to your question other than to say it is fully configurable. The example will as you indicate use the NRF_SDH_BLE_GATT_MAX_MTU_SIZE you set for both UART and BLE packets, but this is just the way is setup. You are free to modify the project and use any combination of size, on any layer of the protocol, you may even split and reassemble packets in the application if you like.

Best regards,
Kenneth