SN75176B: Can I parallel two RS422/485 transceivers like this?

You must not connect two active push/pull output pins together. This happens for the R pins. Either replace U17 with a transceiver that has an RE input (and ensure that at most one RE is high), or combine the two R outputs with an AND gate, or insert open-drain buffers (e.g., SN74LVC1G07) after each R output.

Please note that the SN75176B is very old; modern transceivers like the THVD1520 use much less power and have better ESD protection, and devices like the THVD1426 have automatic direction control.

Hi Jui,

So in general RS-485 transceivers are electrically compatible with the RS-422 standard but RS-422 transceivers are not electrically  compatible with RS-485 standard - this is why we list our devices as as RS-485 and RS-422 as our RS-485 transceivers should work in RS-422 applications. The one parameter that you  should look for in the datasheet is the Vod with a 100 Ohm termination (as in RS-422) and if the minimum is 2V then it should be okay to use. TI doesn't have many RS-422 only devices so most devices from TI used in RS-422 devices actually may be labeled as RS-485. For more information on some of the differences we do have a good app note on the subject here: https://www.ti.com/lit/an/slla070d/slla070d.pdf?ts=1652121019449&ref_url=https%253A%252F%252Fwww.google.com%252F#:~:text=Overview%20of%20RS%2D422%20and%20RS%2D485%20Standards,-Officially%2C%20the%20RS&text=RS%2D422%20is%20specified%20as,in%20Balanced%20Digital%20Multipoint%20Systems.

One thing to note is that the A/B and Y/Z pins are not limited to only RS-422 - devices with 1 dedicated TX path and 1 dedicated RX path are listed as "full-duplex" on our RS-485/RS-422 page: RS-485 TI. Based on the design it seems that the RS-485 is going be on a half-duplex bus (meaning that RX/TX share the same bus and the transceiver can either receive or transmit but not both at the same time. The RS-422 bus in the application seems full-duplex (full duplex is required for bi-directional RS-422 since its a single driver topology). 

So there seems like there are two possible solutions that will  keep the IC amount the same or possibly lower to a single chip - but that depends on a few things.

Case 1: 2 Chip Solution - Shared RS-422 and RS-485 Bus

This solution can be used when the incoming RS-485 bus in half duplex and the RS-422 is full duplex. If the incoming RS-485 bus can be full duplex please see case 2. 

In this solution 2 half duplex RS-485 transceivers, such as the THVD1520 or similar devices with control pins for the transceiver (/RE and DE), act as the full solution. The transmit path (Y/Z on original design) is handled by 1 transceiver - since this transceiver is only used to transmit RS-422 data you can attach a pull-down resistor to R and pull-ups to /RE and DE to keep the bottom transceiver in drive mode at all times. In cases of RS-485 use case DE can be controlled to shut off the driver in case where only the top transceiver is being used (for RS-485 inputs).

The top transceiver will act as the receiver for RS-422 as well as handle the entire RS-485 communication. When in RS-422 mode /RE should be held low and DE held high. When in RS-485 mode /RE and DE should be 0 when receiving RS-485 data and 1 when transmitting RS-485 data.  

Some notes on this implementation:

1. A 120 Ohm resistor is used instead of 100 Ohms as in RS-485 mode both sides of the bus should be terminated and 2 100 Ohm resistors in parallel is 50 Ohms which is below RS-485 spec (minimum is 54 Ohms) so using a 120 Ohm resistor fixes this issue. Most transmission lines have characteristic impedances between 100 Ohms and 120 Ohms so you should still be able to mitigate reflections with a transmission line of ~120 Ohms. RS-422 defines a minimum of 100 Ohms differential and as long as the characteristic impedance of the transmission line is about equal to termination a termination impedance of 120 Ohms should be okay for RS-485.

2. R will be disabled on the bottom transceiver, so high Z, best practice is to tie to ground through a pull down resistor. /RE should be held High always as this device will only be used for transmitting RS-422 data. D is shared as in original design.

Case 2: 1 chip solution (only if incoming RS-485 bus can be 4 wire instead of 2 (full duplex vs. half duplex)

If the incoming signal from the RS-485 bus can be 4 wire then a 1 chip solution may be possible using devices such as the THVD1442 or THVD1552 (full duplex RS-485 transceivers with control pins /RE and DE) or you can use devices such as the THVD1441 and THVD1551 if you don't need to be able to turn off and on the driver/receiver and just want them always on. 

This can be 1 solution if the incoming RS-485 bus can be 4 wires - based on the original design I am not sure if this is possible, but depending on what control you have over that this is a possible solution.

Essentially there is only 1 R and D line that will be controlled depending if a device with controls is utilized. The receive bus will have a 120 Ohm resistor to be in compliance with RS-485 and RS-422 (above min resistance for both standards). The transmit bus will not have a termination - which is against RS-485 standard - however since the bus is transmit only most of the reflections should be mitigated on the receiver attached to the transmission line - however there will be some minor reflections that are reflected back to the transmitter in RS-485 mode - but this shouldn't be a huge concern.

Essentially option 2 is the most simple as it can support both protocols with 1 chip - however it requires that all communication for RS-485 and RS-422 be full duplex - if this is not possible please see case 1.

If you have any other questions please let me know and I will see what I can do!

Best,

Parker Dodson