SN65LBC179: Technical insight into SN65LBC179QDR

HI Lokesh,

I will preface this by saying if you need an RS-485 compliant part do not pick this part - it is only compliant with RS-485 standard between 0C and 70C - outside of this range it is not compliant. Basically any part from this century won't have this problem.  Also if you are looking at automotive stuff - CAN is way better than RS-485 for automotive - but it looks like it may be a speed issue so you can't use CAN which is much slower than RS-485 can be. 

For your direct questions:

1. The driver is maxed out at +/-250mA during short circuit - this is direct from standard. During typical operation 60mA - 80mA is more generally what you would expect from a driver - however that can vary.  RS-485 receivers are rated on unit loading - i.e. the leakage current is going to vary with input voltage based on that of it unit load rating. This device has a UL of 1 - which means at -7V input you can see up to 800uA being sourced from receiver pins and at 12V input you can see up to 1mA being sunk into the pin - this device also includes specifications when VCC = 0V - which you can see a total current of up to 1.2mA in those cases. Generally speaking for RS-485 it is unnecessary and a waste of time in most cases to care that much about specific leakages - general rule of thumb is to convert the unit load to a estimated "worst" case impedance to ground - which is 12k to ground - i.e. we generally suggest 1 UL devices be treated like A is a 12k resistor to ground and B is also a 12k resistor to ground - while not necessarily 100% accurate - when it comes to designing systems  with RS-485 it is easier to just treat 1UL devices as resistive loads at 12k (modern 1/8th UL devices would be at 96k) 

2. Single ended doesn't matter on this device - if you have a single ended application you shouldn't be using RS-485 as forcing it single ended removes basically all the benefits of the system. We only spec differential - RS-485 requires a minimum of 1.5V across 54 ohms  - this device is typically at 2.2V, from 0C to 70C it will be within spec, from -40C to 85C it could be out of spec (1.1V across 54 ohms) - that is why if you look at the datasheet the "75" variant of the part is always in spec and the "65" variants are not - because they are the same parts one of them is just qualified for a wider temperature range - so outside of the commercial 0C to 70C range this device isn't compliant with RS-485 - so if that is a requirement this isn't the right part. 

3. This is advertised as a 10Mbps device - so no it can't do 30Mpbs - you could stretch it to like 15Mbps max and that would be pushing it. RS-485 devices can support 32 unit loads, the worst RS-485 devices are 1UL so you can still fit 32 of them. However the speed is the issue - the 10m may be somewhat challenging at 30Mbps - so you need to have proper cabling too. 

4. Daisy chain is always preferred with RS-485. You should never use a STAR network. 

5. It is general RS-485 basis - they should know the standard it is only like 30 pages and a lot of that is pictures - like the standard is super easy to learn and I would highly encourage you and your customers learn it if they are going to be using it. 

For a very basic overview - please have them read this:

https://www.ti.com/lit/an/slla272d/slla272d.pdf?ts=1770658405613&ref_url=https%253A%252F%252Fwww.google.com%252F

With all that being said since you can't use this part for your application because of the speed  - if you don't need auto-qualified (Q1) parts - the THVD1452 is essentially very similar to the part you are looking at but better - it is fast enough to support 30Mbps, it is fully compliant with RS-485 standard from -40C to 125C ambient temperatures and it still operates in the same way as the older device. 

If you need auto-qualified parts:

1. Do you need them to be full duplex ? 

2. Are there any other needs for the application (power supply etc...)

CAN is usually the ideal choice for automotive - but at 30Mbps that is too fast for CAN so I want to better understand what you are trying to do - because the part you are looking at won't work. If you need a full-duplex Q1 device - what you are looking at is the closest from a single chip perspective - however a multi-chip solution may be possible - I just want to better understand what you exactly need. I will say we may be a bit limited as we don't invest in automotive RS-485 really as that is mainly CAN - but we do have older auto qual'd parts that are better performing than the 79 - but they are half duplex and to make that full duplex you would need a multi-chip solution. 

Best,

Parker Dodson

SN65LBC179QDR is the exact part number I have selected. It’s mentioned as automotive grade in data sheet. ( -40 to +125 C)

RS422 full duplex is the strict requirement in our device.

Data rate speed is 1Mbps.

Also, around 6 to 7 Displays are there at the end application which display the data given by our device.

These displays will be located around 12 metres from the device.