THVD1505: P2P drop-in compatibility with THVD1500?

Hi A.

It depends on the system.

The THVD1500 is fail-safe (for open/idle/short bus conditions) while the THVD1505 is not (this is because the THVD1500 has both of its VIT+ and VIT- thresholds < 0V while the 1505 does not)  

The receiver thresholds, and input hysteresis, and output differential voltage are all different between the devices:

1505 has a higher output differential voltage (2.5V vs. 2V typical) - this shouldn't make too much of a difference as the 1505 is better here. 

1505 has different rx thresholds (VIT+ = 60mV(typ) and VIT- = -60mV(typ) for 1505 while 1500 has VIT+ = -70mV(typ) and VIT-  = -150mV(typ)) - 1505 not protected for bus short/idle/open conditions - external passive fail-safe resistors can remedy the idle/open condition but not short condition) 

1505 has a larger hysteresis curve (Vhys = 120mV(typ) for 1505 versus 50mV for 1500).  The 1505 is more noise immune - but the lack of fail-safe thresholds must be accounted for in the system.

So both the THVD1500 / 1505 and the the other devices that you listed are all compliant to RS-485 standard and half duplex devices with enables are very commonly in 8-pin SOIC packages with an industry standard pinout - however the features they have, the electrical specifications, and timing specifications could vary from part to part so while still compliant the IC's are necessarily 1:1.

With respect to speed - a higher speed device will always transition faster than a lower speed device - so if replacing a slower device with a faster one (even if you have the same data-rate in both IC's) the faster device will transition faster and therefore have more higher frequency energy content which could prove to be an issue from an EMI/EMC perspective. If that's not a concern in your system it shouldn't make a difference. 

When looking at a device to see if you can put a newer RS-485 device on an old board a few questions to ask are:

1. Are they pin compatible

2.  Are the VIT+ and VIT- thresholds different on the new device - if yes - from a system level is this going to cause issues i.e. will the change in thresholds change the performance of the system - this should really shouldn't be that large of an issue as RS-485 compliant systems should have +/-200mV on the inputs - but in some edge cases this could be a problem.

3. Is the driver outputting a lower differential voltage than the last IC?

4. Are there external resistors (for fail-safe in idle/open conditions) - are they sized correctly with respect to the new thresholds.

5. Is there a speed difference between parts?

Essentially for these parts (missing information in bold) :

1. 1505 is pin compatible with 1500

2. VIT+ and VIT- are different with 1500 being fail-safe and 1505 not being fail-safe. What voltages (magnitudes)  are you expecting minimum for a logic 1 and logic 0 in your system - this will determine if the VIT+ and VIT- will cause issue. Is the system spec'd with short-circuit glitch protection in mind? 

3. No - the 1505 has a higher output voltage shouldn't be an issue. 

4. Are external fail-safe resistors present (Pull-up on A and Pull-down on B?) in your system?

5. Yes the 1505 is faster which could lead to more high frequency energy - however the difference between 1mbps and 500kbps usually isn't that concerning. 

In general this will be a similar guide on how to look at different RS-485 parts - they are typically very similar to one another but there are some slight differences that make them not 1:1 with one another.

If you could let me know about the bold text above I can give you a more solid "yes" or "no" answer - but right now it depends on a few system level specs if they can be considered compatible. 

Best,

Parker Dodson