As with any RS-485 (https://en.wikipedia.org/wiki/RS-485) system, the last PL device needs to have a terminating resistor fitted to its output port.
Some early versions of PL modules (eg. PL-3) are shipped with a 100ohm resistor already fitted.
Leave this fitted only if the unit is the last in the chain.
Later version modules are instead supplied with a terminating RJ45 plug.
(Allen & Heath part number 003-082 - spares available through normal distribution channels).