A few years ago, when the local virus made their glory, they were many-many times infected my files and disks. To prefented the virus from infected my files and disks, I always used antivirus program. But because the antivirus program was made by foreign company, many local virus cannot detected by this program. So, to detect the virus in the short time, I used this simple circuit to warning if some DOS tasks made an illegal writing, eg.: when reading a file, program or disk virus always writing their routine to the boot record disk (if it was a boot virus/Gen-B virus) or to the execute program (if it was a file virus). But this circuit also warn if the right task was doing. The circuit in ORCAD for DOS format ver. 1.23. The circuit can also be obtained below in GIF format.
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Diagram block of writing alarm |
The idea is to catch the write enable pin in disks controller card. When this pin logic in high state, the buzzer will ring and the led lamp will glow. In my IBM PC/XT compatible, the write enable pin locate at the 6th from the bottom count (my card controller only for diskette drive, not include hard disk). If you don't know the exact pin or if you have different card controller from our discussion, you can make a try from each pin. It doesn't make any damage at all, until you get the right pin. It can be implemented in PC-AT too. But the right connection pin must be trial. Here is the complete circuit.
Writing alarm circuit diagram 
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