Dear team,
We are planning to use SN65HVD1781QDRQ instead of SN65LBC176QDRQ1. We have seen that drive current is 38mA compared to 60mA in SN65LBC176QDRQ1.
We understand that this will affect the max cable length. What would be the max cable length if we use SN65HVD1781QDRQ with 38mA?
I kindly request you to let us know if there any other impact apart from max cable length.
Hi Mayur,
So the SN65HVD1781-Q1 has a recommended output driver current of +/-60mA - not 38mA. Please see direct datasheet excerpt below.
The amount of current drawn by the device depends on load - the device will only send as much current as load requests (up to short circuit current limit)
Max Cable length is dependent on bus loading, network topology, data rate, and other system parameters such as cabling and layout - output current is not used to approximate max bus length for RS-485. Ideally in a system that is designed in compliance with RS-485 and the system is properly impedance matched it will mainly depend on data rate. So essentially I can't give you a number for max cable length because its highly variable on many factors - but impedance matching is the most important aspect. Ideally the max length of RS-485 is usually given as ~1.2km at 100kbps using 22AWG 120 Ohm twisted pair cabling with proper impedance matching - however many factors will cause shorter buses.
Please let me know if you have any further questions on this topic or any additional questions and I will see what I can do!
Best,
Parker Dodson
Dear Parker,
Thank you for the feedback and detailed explanation.
The drive current 38mA is @ 5V.
We are looking onto port from SN65LBC176QDRQ1 to SN65HVD1781QDRQ. This was one change which we observed and wanted to check on this.
Also, please do let us know if there is any impact which is expected from this IC changeover.
Hi Mayur,
So first off - I just want to preface this by saying the typical characteristic plots listed in the datasheet should not be used for design information on RS-485 device applications as these show 1 device typical behavior - they give an idea of how the device will behave but they can/will deviate from these graphs slightly- but always design with respect to values listed in our parameter tables.
That being said - this is the typical driver output current when you have a 54 Ohm Load on the bus - which is equal to about 2V (our typical listed VOD value for this device). But it can provide more current than 38mA
The SN65LBC176QDRQ1 has lower drive strength than the SN65HVD1781QDRQ.
The major factor that you can look up and compare is the output differential voltage |VOD|.
SN65HVD1781-Q1 will hit a minimum of 1.5V across the resistor (across temperature range) when the VCC range is between 4.75V and 5.25V (the same supply range as the SN65LBC176Q)
The SN65LBC176Q:
So across temperature the SN65LBC176Q only guarantees 1.1V across the resistor whereas the SN65HVD1780-Q1 guarantees 1.5V across the resistor - this means that SN65HVD1781-Q1 has greater drive strength.
If you are considering the SN65HVD1781-Q1 to replace the SN65LBC176Q you should be able to communicate at the same distance - if not longer - as the older device you are using is not actually compliant with RS-485 and while the device you are considering also isn't fully compliant across supply range - it is very close to compliant at 5V.
With all this being said however the biggest issue you may face is with data rates. The SN65HVD1781-Q1 is only rated to 1Mbps whereas the SN65LBC176Q is rated to 10Mbps. So if you switch devices your applications needed data rate must be capped to 1Mbps.
Please let me know if you have any other questions!
Best,
Parker Dodson
