This TTL to USB serial converter cable had been in used for many years. It could no longer receive, but still could send. A loopback test between TXD and RXD got no response. A resistance measurement between RXD and any other pins showed open. I cracked open the USB connector to expose the circuit board and confirmed that there was no open in the wire. The circuit board is just a FTDI FT232R chip (3Mbaud, FS USB) and a few passives. The date code of the IC is 1344-C, which is 44th week of 2013, rev C. FT232R is not recommended for new designs by FTDI. The output signals have 100Ω in series; the input signals go straight into the IC. This version of board has 3.3V output.
I checked the solder joint which appeared no different from the others. I probed directly at the IC pin and found the RX pin was still open. It could be an open bond wire or a metal trace on the IC. It's hard to image a failure of transistors could leave it complete open. It could be caused by an ESD event. The datasheet does not give any information on the ESD protection level; one app note says the FTDI IC's are tested to 2.5-3kV, not sure which (HBM, CDM, or MM). I suspect it is the HBM, which would make it Class 2, a little low for interface, but the IC is intended to work with other line drivers.
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