I have implemented a USB to RS-485 converter using the FT232RQ from FTDI, based on their Figure 7.2 ("Application Example Showing USB to RS485 Converter") in the FT232R USB UART IC Datasheet. The level translator I use is an ADM485EARZ from Analog Devices. The CBUS signals are routed as in the Figure 7.2.
Sadly, it does not work as intended, and I'm trying to find out why.
The data from the USB input through the FT232RQ and the ADM485EARZ seems to be in order. However, the data I read from an RS-485 device (a laser controller) is plagued by intermittent errors.
I can see that there's a wrong checksum when an error occurs. Typically, I read 10 bytes from the RS-485 device. Of these the first 4 bytes are always correct, but errors may then occur in the bytes to follow. It also seems that I read one more byte than expected when an error occurs. The error rate is about 2 % or so. The RS-485 signal appears to be in order with no strange levels or overshoots, etc.
Anybody got a hint on what to look for?