RS485 network failing on replacing SP485 chip with MAX485
Do you have pull-up/pull-down bias resistors on the lines?  These bias resistors set the state of the line when it is idle, i.e. no devices are driving it.  The behavior of the driver chips is undefined by the standard if the lines are allowed to both float to zero volts.  Some drivers implement a feature called 'fail-safe' such as the MAX485, with this feature bias resistors are not required.

See Failsafe Bias Resistors section in this application note.