I2C DAC

Set a voltage:

sudo i2cset -y 1 0x60 0x40 0xff 0xf0 i
                             ^^   ^     < Voltage bits

explanation of the arguments:

-y    don't ask confirmation
1     number of the I2C bus (0 on older raspberry pi's)
0x60  address of the DAC, use 0x61 for the other DAC. 
0x40  configuration: normal. 
0xff  high 8 bits of the value
0xf0  lower four bits of the value (in the high nibble)