5i25-7i77+7i74

I just checked the cables . The one I had doubts about has every connection right, but I can't check the twisted pairs . The one I thought is better is missing pin 12 . Both dont work . Today I changed out the 5i25 and 77 . The problem remains . On the other hand I think the other boards are fine . Where can I buy good cables?

You should use a IEEE-1284 compatible cable

Also check the 7I77 and 7I74 5V.

Marginal 5V on the 7I77 especially is the most common cause of sserial problems

This is why its suggested to power the 7I77 with an external 5V supply rather than via the IEEE-1284 cable
(you need to change both 7I77 and 5I25 jumpers to do this)

I use an external 5v power source . Supply voltage was one of the first things I checked .
About the cables .
I'll search around for twisted pair encoder type cables with enough wires in them . How should the twisted pairs be soldered to the DB25?

I still haven't got to that point yet . It hasn't worked for more than 10 min and the vfd is not even powered up then .

The complete failure of the sserial communications point to some systemic problem
I would check your power (including ripple) and 7I77/5I25 jumpering again

I would also try running with no external connections on the 7I77 as you may have a severe ground loop issue

The 77 is disconnected . Power seems good . Ripple is within 40mV which should be fine . Even without any connections to it, serial still fails . Halcmd shows the read function . I connected the 5V and 24V grounds . In the original wiring the 24V gnd is connected to PE on the machine which in our case is also neutral on the three phase supply .

P.S. I reflashed it with 5i25_7i77_7i74.bit and now it works . Any ideas why it didn't work with 5i25_7i74_7i77.bit? The cards were pluged in the correct ports .

Not sure, might still be a cable issue.
Without a IEEE-1284 cable, all bets are off

I shortened the cable by about a foot and it was surprisingly well made . That didn't improve things but the firmware did .
The first one had the 77 on the expansion port and the 74 on the onboard db25 . The one that runs has them the other way round . Is it possible that the ribbon cable was the problem?