how to write text to a serial port in simulink

I have a device that i want to communicate with using Matlab. it uses a serial port to connect to the computer. i want to use matlab to write the word "setdo1" into the device. the device will recognize it and replies. I have done this using a matlab code successfully. the problem is I cant do this using SIMULINK. because it seems that simulink doesn't deal with string variables. i dont know how to do it with simulink for example i want to run an algorithm like: if a=1 then fprintf(s1,'setdo1') in matlab i got no problem with this. but in simulink i don't know how to do it. please help me