Thank you for the reply.
In my experiment, I want to have two kinds of trials: CS_MINUS1 and CS_PLUS1. In CS_MINUS1, a visual stimulus is presented for 6s; in CS_PLUS1, another visual stimulus is presented for 6s and, but in its last 0,2s, there is an electrical stimulus that is presented simultaneously. That is why I programmed the second visual stimulus to be presented for 5,7s, followed by the digital output 2 that produces the electrical stimulation.
When comparing the durations of CS_MINUS1 and CS_PLUS1, I noticed the first trial was presented for 6s, but the second was presented for approximately 5,8s, that is why I was under the impression the visual stimulus was erased before the digital output. You are right, the picture does stay on screen during the digital output. I think I found why I am having problems. I programmed the digital output to end immediately after its presentation and that is why the screen turns black. I thought that using the trial pacing of 6s would solve the problem, but I forgot that the trial contains the inter-trial-interval (i.e., the event showing a blank text that uses a variable time limit).
I think the solution is to turn off the the trial pacing feature and program the digital output 2 to end after 0,2s after its presentation. This way I think both CS_MINUS1 and CS_PLUS1 will have the same duration.