I see two points: MAX3291 isn't specified to extent RS485 at low baud rates above 1200m. It's specific preemphasis feature isn't effective for low baud rates respectively very long cables.
On the other hand, the specified RS485 1200 m limit is a convention without clear technical foundation. It's clear, that considerably higher distances are feasible at low baudrates.
Yes, I've seen the diagram. If you look at the preemphasis timing, it involves a 100 ns current raise. So the effect is reduced below 5 Mbps and neglectible below 1 Mpbs, I think. The diagram was made by connecting two point with a straight line or drawing a parallel line through one point.
But if the diagram is meaningful at all, you could extrapolate the conventional driver line to 100 kbps and below.
The diagramm says 800 kbps for the MAX3291 and 400 kbps at 1200 m respectively 4000 ft. As I explained, the MAX3291 value is most likely an incorrect extrapolation.
The diagramm says 800 kbps for the MAX3291 and 400 kbps at 1200 m respectively 4000 ft. As I explained, the MAX3291 value is most likely an incorrect extrapolation.