RS485 Full Duplex need star topology any suggestions

i have design RS485 Full Duplex with daisy chain topology but my boss need star topology

Please give me any suggestion the max cable length from Master to very first node id 3 meters and from slave to slave is less then meter.

Current Wiring


Baud rate : 9600
4 wire
MAX485 IC used


Master : ATMEGA2560
Slave : ATMEGA328P

But My Boss need :

Each cable comes from Master (MASTER<—3meter—>SLAVE) to slave as on star topology.

My Question is this way can RS485 full duplex will works perfectly.

or is there any why to do the same either UART / SPI / I2C

I dont know it works or not:
as this is Big Segment Display by using 74HC595 and can we use 3 meter data cable from master to slave.
Like :

MASTER with 74HC595 <-------------3 meter ribbon cable --------> 74HC595 on display directly. or with other IC.

Please suggest me…


Jasbir Singh

Welcome to the Arduino forum. However, if you have researched RS-485, you know by definition it it HALF-DUPLEX. Cannot be otherwise.

But you are actually defining TWO RS-485 networks. A star topology will be no different than daisy-chaining slaves one after the other. Just a LOT more wire.


Thanks Paul, but as per our project my Boss need star topology either its UART / SPI or any communication method. that way i am confused, you know BOSS is always right....

Please suggest me any other way to do the same.

At star is quite different than daisy chain.

PDF from TI on RS-485

Page 5 shows various cable topologies. C is what I know as a star. B, D, and F are daisy chain.

Star is a no-go in RS-485. The drop lengths will be so long as to each require a terminating resistor but as there can only be two terminating resistors in the system that won't work either. Star is dead for RS-485.

Show you boss page 5 and have him choose from the right column of choices, because the ones in the left column won't work.

@ Paul_KD7HB RS485 full duplex is a reality (however the half duplex version is more frequent ):


A CAN bus star topology ?

@ Paul_KD7HB RS485 full duplex is a reality (however the half duplex version is more frequent ):
MAX13448E ±80V Fault-Protected, Full-Duplex RS-485 Transceiver | Maxim Integrated

Oh, sorry!

I was referring to the AT&T. Bell Labs Recommended Standard #485 documentation(AKA RS-485).


Dear All

Please suggest me the why how i can design new communication from MASTER to SLAVES on different Serial Communication method which will work on Star Topology.

Brief on Project Big Segment Display :

Our project is One Master can send command to display message to any unique slave (Max slaves 10) and receive ACK from slave.

To run display we have used 74HC595 IC SPI interface.

What does the SPI have to do with the project?

As for available star master-slave topologies that look like a star, that depends on distance.

For any decent distance, I can think of two. Ethernet and wireless.

You can make a star into a daisy chain (or a daisy chain look like a star) by running from point A to point B, then back towards point A (but not connecting to point A) then out to Point C. So each leg has two cables. It will look like a star but actually be a daisy chain.

Brother : distance is 3 - 4 meters from Master to Slaves.

Is the system already working today as a daisy-chain topology?

If YES, then we need to understand the addressing you use to select the remote targets.
If each node performs selective pass-thru, that can/will affect the conversion to star topology.

You say the remotes are only 3-4 meters from the host - that seems pretty close!

3-4m ribbon cable, I2C, SPI, TTL are not going to work over that distance.

What problem have you been having that the boss wants to change to a star topology? I can imagine, but want to know if it is the same as I suspect.