There was once many people making IrDA transceivers, back when they were used in Cell Phones before the era of Bluetooth, now there are only a couple of manufactures left. Once I had three sources for a footprint compatible part with the HP3003 that I originally started with. Over the years the only remaining footprint compatible part was the Vishay TFDU4300.
Sadly the day came when even the 4300 was discontinued, however I was assured that I was not going to have to do a new board spin, "you can drop our new TFDU4301 right in". While true that it was footprint compatible, it was not software compatible. The 4300 would not receive its own output and would drive its Rx line high during transmit. The 4301 echos the transmitted data, meaning all of the bootloaders had to be updated to turn off the receiver when transmitting. Annoying, but at least that was documented.
What was not documented is shown in the scope captures below. The first one shows the discontinued 4300 and the second shows the new 'drop in compatible replacement'. Does it look compatible to you?
Sadly this problem was discovered while I was far away trying to get my wife help with Intracranial Hypotension, more commonly known as a Cerebrospinal Fluid (CSF) Leaks, to stop her excruciating headache resulted from cerebrospinal fluid leakage. I made the mistake of putting my job first and we came home to Pittsburgh, when we should have stayed at Duke in North Carolina. This is one of the small events that accumulated to result in my wife's suicide.
Now does 'drop in replacement' strike fear into your Soul too?