Sensor Network

You will need to determine if your gateway can handle multiple addresses on the RS485 bus.  My guess would be that it supports a single node.

Thank you! I was also worried if the gateway can do that. If I go for a better  gateway as in here https://www.alibaba.com/product-detail/Industrial-Gateways-Modbus-RTU-TCP-Master_1601044443084.html?spm=a2756.order-detail-ta-ta-b.0.0.54f82fc2Uya6yU,

With the hub, will I be able to connect multiple sensors?

corel_draw Please forgive me if I start out too basic.

I started learning this stuff by researching MODBUS RTU.  MODBUS and RS485 are not the same.  My simple approach may offend some, but... crawl, walk, run.  MODBUS is the language.  It's not that important right now.  RS485 is the hardware - an old school telephone party line your information will travel on.  You can wire one sensor to the next to the next in most cases.  Each node on the line gets a different node address (1, 2, 3...).  Your RS485 "network" will probably be Master/Slave.  The Master asks the questions.  The Slaves answer.  The Master calls the Slave by the Node Address.  "Node 3:  What data is at data point 0x0010?"  Only Node 3 answers.
When you wire it, A is always A.  B is always B.  For everyone.
Put a 120 ohm resistor (only 1) at the very end of the line to dampen any "bounce-back" of the transmission.  I liken this to a LONG hallway and all the nodes are rooms along the hallway.  If there's a lot of echo in the hallway, you'll have a hard time hearing when your node number is called.  Hang a blanket on the wall at the end to dampen the echo.

Where Gateways come in is if you need two or more Masters asking questions.  Now we need a traffic cop.  Your turn.  My turn.  His turn.  Her turn.  Repeat.
OR if we need to change transport media - say going from our little simple wire party line to Ethernet.  This is your application.
You should be fine starting out with Gateway -> Device -> Device.

You will also run into MODBUS TCP/IP.  This is a bit of what your Gateway will be doing - sort of.  MODBUS TCP/IP simply takes the MODBUS RTU data packet, wraps it neatly in an ethernet TCP/IP "box" and puts it on the ethernet network.  That network does it's thing and delivers the box where its supposed to go.  One the other end, MODBUS TCP/IP pulls the MODBUS RTU stuff out of the box and either processes it or plops it onto a different simple party line RS485 network.

I hope this helps.

EXCELLENT! Great high level intro  kmikemoo  .

Terminating the end with a resistor is a valuable tip. I recall first learning about scsi. The instructor had this complicated description of terminations. I spent almost a lifetime in RF. Understanding that you are terminating the end of a transmission line to eliminate reflections, caused his complicated description to disappear.

I'm curious about your 120 ohm value. I've used 600, 50 and 75. 120 is a new one on me.

 colporteur I have no idea where the value came from - but we also use it for CAN.  In  teaching basic CAN or RS485 troubleshooting - with the system powered down - check the resistance between A and B.  It should be 60 ohms.  120 ohms from the Master (in most cases but not all) and 120 ohms at the furthest point of the network/cable run.  NO star topography.  Those little branches can become stub attenuators. 

I never played in that space so the values would seem foreign to me. As soon as you mentioned transmission you brought back knowledge gained 30 years ago. Thanks for the trip down memory lane.

https://kvaser.com/developer-blog/how-to-test-your-can-termination-works-correctly/#:~:text=It%20is%20well%20known%2C%20in,each%20end%20of%20the%20bus.

I found this on quick search of CAN network termination. Wow at 63 I learned something new. Now if I could only remember it:)

colporteur I didn't understand the problems with star topography until I got into Amateur Radio about 6 years ago.  Now it all makes sense.

It's not surprising that you found Kvaser.  They make many of the CAN readers/programmers used around the world.  I've used their stuff on German, Swedish, Japanese, Italian and American equipment.  They like customizing their stuff for the different OEMs.  We have 3 readers that look exactly alike but will only work on their respective engine lines.

CAN-B (83.3 Kbps fault tolerant) and CAN-C (500 Kbps) both have 120Ω dominant node termination. CAN-D has 60Ω. Recessive nodes have 3kΩ terminations.

CAN-B (83.3 Kbps fault tolerant) and CAN-C (500 Kbps) both have 120Ω dominant node termination. CAN-D has 60Ω. Recessive nodes have 3kΩ terminations.