(Paraphrased from the DirectRT Manual

For DirectRT's TTL receive routines to work *at all* in an experiment, you first need to transmit a byte of data from that same machine. It is okay nothing is connected to the computer to receive this data.

Without that first transmission, the receive routines won't work properly.

If you're able to send and receive data using the IOTest utility, we know
that the cable is good between the machines.

So, at the very beginning of the experiment file on your receiving computer,
insert the following lines:

stim | loc | time
ttl:1 | 888 | 1000

This will send a 1 second pulse and properly initialize the receiving
computer.

Then, you can continue using the experiment you've written.