All of the planters I have seen built prior to 1990 have some sort of chain drive system, getting the power from one or more of the planter's wheels. They all have three things in common.
1. There is a clutch to disengage the planter drive on the headlands. On tractor pulled units the clutch is linked to the raise/lower system so the drive is disengaged when the planter is raised and engaged when lowered. I suspect horse drawn versions used a rope pulled by the operator.
2. There is some way to vary the plate speed relative to ground speed. This was usually accomplished with a "transmission" made from multiple interchangeable sprockets that could be easily swapped giving many different ratios. There would be a chart in the operator's manual (and often on the machine as well) that would list the different ratios, and given a certain number of holes in the plate and row spacing, the seeds per acre. I have also seen gear clusters or sprocket clusters that could be slid on a shaft, and you could line up the two you wanted to use. This was an improvement since the sprockets couldn't get lost!
3. Many planters also have some type of one-way clutch to allow the planter to go in reverse without turning the meters in reverse. This may be handled by #1, which would require the planter to be raised before it is reversed. But usually they have one over-running clutch on each wheel, which provides reverse protection and makes the meters run with the faster wheel (which isn't a big deal except on corners.)

ISZ