Searching this issue on the web, I found the comment "Most systems, including x86 GNU/Linux and Microsoft Windows, use signed char, but those based on PowerPC and ARM processors typically use unsigned char."
This presumably explains the behaviour I saw since the 485 is a PowerPC chip.
Am I right in thinking there is an Omron "Delta Tau" product on the way that uses an x86 processor? If so, it seems that we will need to watch code portability here.