Emerson COM Express Carrier Design Manual page 52

three analog color signals should be back terminated on the carrier board to ground through
150 ohms. The signals should be given generous spacing to each other and to other signal groups.
Shorter routes are preferred, although longer routes can work if carefully done. Reference the signals
to a continuous, low-noise ground plane. These signals are also terminated on the module through
150 ohms to ground, resulting in a combined module/carrier board side termination of 75 ohms. The
VGA monitor will also have a termination of 75 ohms. The module electronics are designed to drive the
net VGA termination of 37.5 ohms.
The horizontal and vertical sync signals VGA_HSYNC and VGA_VSYNC provided by the COM Express
module are 3.3 V tolerant outputs. Since VGA monitors may drive the monitor sync signals with a
tolerance of 5 V, it is necessary to implement high impedance unidirectional buffers. These buffers
prevent potential electrical over-stress of the module and avoid that VGA monitors may attempt to
drive the monitor sync signals back to the module.
COM Express provides a dedicated I 2 C bus for the VGA interface. It corresponds to the VESA-defined
DDC interface that is used to read out the CRT monitor specific extended display identification data
(EDID). The appropriate signals VGA_I2C_DAT and VGA_I2C_CK of the COM Express module are
supposed to be 3.3 V tolerant. Since most VGA monitors drive the internal EDID EEPROM with a supply
voltage of 5 V, the DDC interface on the VGA connector must also be sourced with 5 V. This can be
accomplished by placing 100k ohm pull-up resistors between the 5 V power plane and each DDC
interface line. Level shifters for the DDC interface signals are required between the COM Express
module signal side and the signals on the standard VGA connector on the carrier board.
Page 52 of 103 COM Express Carrier Type 2
Design Guide