editing – reading eprom hex dat,

First of all, I am a specialist in computer hardware and I am trying to modify EPROMS to add a test file to check the operation of a control card, lights, inputs / outputs, etc. and a SRAM Dallas NV. My problem is to try to interpret the hex code. I can change the hexadecimal code to change the text displayed on the screen of the machine (start, run, load the product, etc.), but the ASCII does not make any sense, just gibberish. I have tried many Hex editors and disassemblers, but none can convert Hex or ASCII code into anything I can recognize. No advice? Thank you.