[Don]

The VP-X manual offers the following instruction for connecting the two.

Connect the three data wires on the VP-X J1-20, 21, 22 to a serial
data port on the EFIS. Typically serial data transmit and receive
lines are grouped together on the EFIS. The transmit line (J1-20)
on the VP-X goes to the receive line on the EFIS. Conversely, the
receive line (J1-22) on the VP-X is wired to the transmit line on
the EFIS. The VP-X communicates at 57600 bits per second.​

The Hx serial ports have an IN and an OUT (which is how I thought all serial ports are - two wires not three but maybe some have 3 wires). I'm curious what others have done to connect the two devices. My plan is to use J1-20 as out from the VP-X to the IN on the GRT and the J1-22 as the in to the VP-X from the OUT on the Hx. I'm not sure if I need to do anything with J1-21 and I figure someone here has crossed that bridge.

 

[Mike S]

IIRC, the third wire is a ground for the data in/out wires. Some use it, some dont.

 

[MCA]

Don,

You can ignore the serial ground on the GRT. Section 5.12 in the VP-X install manual describes the specifics of the GRT installation:

On the HORIZON, trim and flap position are shared on the display
interlink between display, but the status of each electrical device is not.
The VP-X electrical page only appears on the display that is wired to the
VP-X. On the SPORT, no data is shared between the displays, so wire
the VP-X serial line to the display on which you want to see trim and flap
position and VP-X electrical page.

Use a serial port pair on the EFIS. Make sure the EFIS and the VP-X are
wired to the same ground. Do not use the serial ground on the VP-X.