code: public ProcessImage getProcessImage(int unitId) {
          ModbusSlave slave = ModbusSlaveFactory.getSlave(this);
          if (slave != null) {
              return slave.getProcessImage(unitId);
          }
          return null;
      }
docs: Returns the related process image for this listener and Unit Id
  
  @param unitId Unit ID
  @return Process image associated with this listener and Unit ID