The sample program provided in the instruction manual for the Dual Serial Motor controller is for a Stamp II controller but I have an Atom.
To make it work I had to use this command.
serout 15,i9600,[$80, 0, 1,speed]
It seems weird since the i9600 means inverted at 9600 buad and the motor controller requires no inverted, but it works.
I also used a 10k pull down resistor on the reset pin.