No go
I've pulled the VCC line (USB-TTL to pin 7 on the DCS) and wired an external 5v power supply (2A). So... from the external power supply, VCC on DCS pin 7 and ground on DCS pin 3 (which is shorted to pin 2, according to IGT manual). Pin 2 is still connected to GND in the USB-TTL.
This way, I'm powering the DCS interface independently and have a common ground. I can still receive data (machine SAS address) but nothing transmitted (nothing shows on COMM ANALYZER). The TX LED on my USB-TTL is alive, but the vgm does not react/receive.
When I pulled the external 5v power, I stop receiving, which tells me that the interface is properly powered.
I've tried also using a pull up resistor (750) on the SDO line as instructed by the IGT manual. RX works fine, just to rule it out.
Suggestions are welcome. I'm running out...