SN65HVD78: Termination resistor and connecting more than one slave devices with the transceiver

Sharan Ghimire

Part Number: SN65HVD78

Hello, I am using above mentioned parts to convert TTL /cmos logic to rs 485 and i am wondering if i need a termination resistors and what value ( typically its 60 ohm on each side). Other question I have is: I have 2 slave devices one is connecting to transceiver ( ti part) directly and i have rs 485 to usb converter to connect transceiver to computer. I have also attached a block diagram . can you tell me if the system i have it setup is correct or no? thank you

over 3 years ago

0 Eric Hackett over 3 years ago

TI__Guru 73372 points

Sharan,

An applications engineer has been notified of this post and will respond accordingly.

Regards,

Eric Hackett

0 Hao L over 3 years ago

TI__Mastermind 25705 points

Sharan,

What's the data rate and distance of each segment (from the SN65HVD78D to the sensor, from the RS-485 to USB converter to the bus)? In this app note,

https://www.ti.com/lit/an/slla272d/slla272d.pdf

the detail of stub length vs. data rate is discussed in section 6. SN65HVD78D works with 50Mbps with 3ns rise time. In general, the RS-485 system prefers a main bus with short stubs.

0 Sharan Ghimire over 3 years ago in reply to Hao L

Prodigy 110 points

Hello Hao, I believe distance between sensor and SN65HVD78D will not be more than 3 feet. and the distance between usb converter to RS 485 would be less than that.

Could you also please look at the setup diagram and may be have any suggestion if my setup is ok?

Thanks

0 Hao L over 3 years ago in reply to Sharan Ghimire

TI__Mastermind 25705 points

Sharan,

The setup should work except one note. The 60Ohm termination may need to change to a higher value like 120Ohm, which matches the cable impedance. In section 6 of the app note I shared, different termination method is also discussed.

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SN65HVD78: Termination resistor and connecting more than one slave devices with the transceiver