Assuming that all is OK, a correctly booted SITCOM will flash itís SOD LED at a rate of about 0.7Hz with a 50% mark space ratio if you are using the standard 6.144MHz crystal. With an older 8085 using a 3.075MHz crystal, this will be about 0.35Hz - i.e. twice as slow :) It is important to realise that the value of the crystal determins the baud rate for downloading the program data from the assembler, so anything other than these values is likely to cause difficulties on the comms. front.
If the SOD LED is NOT flashing, try pressing the BOOT button. This is effectively the same as pressing a normal hardware reset button. If you have the two displays connected to your SITCOM, then at BOOT a message will appear on both displays. If nothing still happens, switch off and try to locate the problem. If you are at all unsure about your BOOT prom, might I suggest that you substitute another one containing a short loop to output a value to one of the I/O ports of the LS138? Using something like 3E, 41 ,D3 ,20 ,18 ,FAh, (from 0000h to 0005h) you can easily test for an output pulse with a logic probe on the LS138 pin 14 in this instance and NOTHING on any other I/O port.
When a program is being downloaded from the Assembler, the SITCOM will automatically see this on itís SID line (connected - hopefully via the transistor shown on SITCOM schematic Page 2 to the correct PC comm port) and the LED will freeze in the ON position. until it is all loaded. At that point the SOD LED will flash with a much shorter pulse. If you have the displays connected, a rotating bar will appear until the program is loaded, then the message OK RESET will replace it. On depressing the RESET button, the SOD LED will stay on constantly and - hopefully - the program will run. And that is it!