modbus master and not slave?

The Modbus command in Matlab, part of the Instrument Control Toolbox, sets up communication between the PC running Matlab and a PLC or other device. However, it appears that Matlab is the slave and the PLC is expected to be the Master. Is there any way to make Matlab the Master? My PLC is only a slave, and there must be a way to assert the PC as the Master. How to do it?