• TI Thinks Resolved

ISO1500: RS485 Bus Stubs length regarding data speed

Prodigy 20 points

Replies: 2

Views: 50

Part Number: ISO1500

Hello,

We have this question: If we are not using a bus speed of 1Mbps (ISO1500) and we stay at today 4.8 to 19,6 kbps, can we increase the stub length calculated according to the TI AppNote? > this length is ~5 meters max for Tr/Tf = 210ms typ.

It may impact our Bus system if we must reduce with this new driver our stubs length (10 ~15 meters today)

As this value calculation depends on Trise/Tfall of the transceiver, and in the case of the new ISO1500 (only available @1Mbps max and not 200KBps) this time is quite low. That’s may mean lower length of stub?  I have a doubt and I’m not sure to be right ! May the Tr/Tf will increase if we use this transceiver at lower speeds (up to 19600bds max).

Thanks in advance for your reply and confirmation

Regards,

Michael

  • Michael,

    The rise and fall times are more dependent on line resistance and capacitance rather that communication speed. The driver rise and fall times are speced at a line resistance and capacitance of 54ohms and 50pF respectively.  So based on the equation in the datasheet, then the ideal stub length should shorten as rise and fall times reduce.

    Respectfully,

    Lucas

  • In reply to Lucas Schulte9:

    Hello Lucas,

    Thanks for your answer. All right, So it seems not so easy to quantify in concrete terms of stubs length.. but we can hope to have also proper communications with little bit longer stubs @lower speed. We usually use CAT-5e cable (100R) and 2 termination résistors 120R on the bus.

    Best Regards,

    Michael