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Which is the best IC for RS485 to ttl

surya
Expert 2310 points
Other Parts Discussed in Thread: THVD1426, THVD1406, THVD1454, THVD1420, THVD1400, THVD1410

Dear Experts,

Currently, i have used the below design for RS485 to TTL with auto flow control(74HC), now i have the plan to redesign my converter with ti RS485 converters so please suggest the best design for my requirement,

Supply volt-3.3V-5V

Less BOM 

Automatic flow control(i want to remove 74HC from my design).

regards

Surya

  • 0
    TI__Mastermind 46687 points

    Hi Surya,

    For automatic direction control we have two devices with integrated auto-direction features - the THVD1406 (500Kbps max speed) and the THVD1426 (12Mbps max speed). If /SHDN is held high (there is no DE pin - its a /SHDN pin) and D goes from Low to High the driver will become active for a period of time (1406  min: 4us; typ: 8us; max: 14us : 1426 min: 400ns ; typ: 800ns ; max: 1.45us) - so this feature on this part will only work for a narrow band of data rates. 

    However you can convert any RS-485 (with DE and /RE pins)  device into one that supports automatic direction control - that may be a bit easier to use than the current design. It uses timers to set a frame length: https://www.ti.com/lit/ug/tidubw6/tidubw6.pdf?ts=1677169475846&ref_url=https%253A%252F%252Fwww.google.com%252F the part that is chosen is based on needs of application such as data rate and number of nodes that must be supported on the bus - if you can share data rate and numbers of nodes that must be supported I can guide you onto a more specific part. This method should isn't as BOM friendly as the integrated solution but it does have a bit more flexibility. The link I provided does reference a specific part - but the design can largely be applied to most RS-485 devices.

    Please let me know the application data rate and number of nodes supported or if the THVD1406/THVD1426 will meet your needs!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Expert 2310 points

    Hi 

    Thanks for the reply, Currently i have used 10Mbps and the number of nodes is 256 so please suggest >10Mbps and 256 nodes supported IC, i can use 7414 Hex Schmitt-Trigger for direction control, or if you have any Schmitt-trigger IC Please suggest too...

    Supply Voltage:3.3V

    Regards

    Surya

  • 0 in reply to surya
    TI__Mastermind 46687 points

    Hi Surya,

    The THVD1426 is most likely the best fit for your application.

    It works very similarly to your current setup.


    By tying /RE and /SHDN to logic high the device will be in auto-direction mode.

    1. The device will output on A/B following inputs on the "D" pin.

    2. If the "D" pin is held high for t_device_auto_direction ( 400ns (min), 800ns (typ), and 1.45us (max)) then the receiver will turn on. 

    3. The receiver pin "R" will follow the bus input voltage until "D" is pulled low - when "D" is pulled low the device will switch to drive mode. 

    This works similarly to the current setup - but the mode switch delay is different - you are looking at a mode switch time around a max of 1.4us with the THVD1426 - with the current setup  - you are looking at milliseconds of delay between mode switch due to the charging time of the capacitor in the current schematic. 

    If that long of a delay is needed from driving to receiving in the system than the THVD1426 isn't the correct device. However if this delay can be shorter the BOM can reduced by using the THVD1426  - there is no need for external RC charging networks in that case and:

    1. It can transmit up to 12Mbps (so >10Mbps)

    2. It can work at 3.3V supply

    3. It is 1/8th unit load so it can support up to 256 nodes on a bus

    The biggest difference by using the THVD1426 is that the delay between "D" going high and the receiver turning on is going to be shorter on the THVD1426 . If that delay is too short between mode switches please let me know as if that is the case the current schematic you have will work fine - but there will be a few more options  in device choices so any other features that may be wanted if you could please let me know in case the THVD1426 isn't the best option for you. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Expert 2310 points

    Hi

    Thanks for the detailed explanation, yes you are correct here most of RS485 enabled instruments required a 1sec delay for the next command so i can't go with auto direction control devices.

    Please suggest without auto direction control, here i have one doubt regarding choosing the exact datarate for our application, Most of the RS485 devices have a baud rate of 9600, and the baud rate is not more than 115200, then why do we need to choose an Mbps data rate.

    regards

    Surya 

  • 0 in reply to surya
    TI__Mastermind 46687 points

    Hi Surya,

    Alright - then if that is the case then we have a lot of potential options as this application can fit many different devices.

    If you are looking at >= 10Mbps as indicated by your previous response than we have about ~15 options: https://www.ti.com/interface/rs-485-rs-422/products.html#p1065=Half&p0nom=3.3;3.3&p976max=10;50&p1064=256;320 

    The two options that are probably the best suited - as they don't have as many additional features that are most likely not needed are:

    1. THVD1454 - half duplex device that can have slew rate control to slow down the transition speed if higher data rates are not needed. It can work in this system and also contains an integrated termination resistor that will allow you to not need to add it to the schematic externally. 

    2. The THVD1420 is a small package RS-485 device that should meet the needs of the application without much issue.

    If you don't need >10Mbps and are looking at only 115.2 kbps max speed than that opens up the options a bit: https://www.ti.com/interface/rs-485-rs-422/products.html#p1065=Half&p0nom=3.3;3.3&p976max=0;50&p1064=256;320 

    But two additional options besides the ones listed above are:

    1. THVD1400 - 500kbps max device

    2.THVD1410 - 500kbps max device 

    Both of these devices should meet the application needs but are slower than the 10Mbps that was originally indicated.

    You don't need higher speed than necessary - usually its not preferred to use higher speed devices for lower speed needs as there is more high frequency harmonic content due to faster transition times on the driver outputs. This may or may not add negligible risk as it will depend on system configuration and setup. 

    Any of these devices should be fine - the application you have isn't too sensitive to exact IC if you can't use the auto-direction feature. But the parts listed above give a few good solutions that should work well in your system.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Expert 2310 points

    Hi

    Thanks for your guidance i would like to choose THVD1450DR for our design, please share your opinion.

    Regards

    Surya

  • 0 in reply to surya
    TI__Mastermind 46687 points

    Hi Surya,

    The THVD1450DR should work in the application. The one note is that is is rated up to 50Mbps so the transition times will be faster compared to a slower device when ran at the same data-rate. In a properly impedance matched system this really isn't that much of an issue - but it something to be aware of.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson