The Canning Machines (both regular and vacuum) won't accept items from outside sources, including hoppers, pipes, and other machines trying to export to them. Is this a problem with the machine having two input slots?

I tried designating the slots using the inventory/slot upgrades but they are a little confusing (including the simple Automation Upgrade), and it still wouldn't take in items.

Here's are some screenshots:

@AtomicWaffle14 the slot is by default only accessible from the East. (It rotates also with the machine so its the right side when you look at the "Front" face. Cells come into there by default.

Also i assume you placed the "Inventory Upgrade" (Automation Upgrade) into the "Machine Upgrade" Slots (where overclockers can go)
If thats the case then thats the reason why it didnt work. Because Inventory Upgrades have their own 5 Upgrade slots that can't be viewed normally.
They have to be accessed with the machine tool.

The strange this is that this is not happening with any other multi-slot machines, such as the Uranium Enricher.

Trying putting the food in from the top and the secondary into the EAST side. automation is designated per side

Trying putting the food in from the top and the secondary into the EAST side. automation is designated per side

Tried it, doesn't work.

ok tried it, yeah its weird. it wouldnt start "hoppering" apples in until I placed a tin can in the bottom slot. and wont accept input for the cans into any side.

@AtomicWaffle14 the slot is by default only accessible from the East. (It rotates also with the machine so its the right side when you look at the "Front" face. Cells come into there by default.

This is actually not true. It is only accepting cells/cans from the back, and other ingredients from the top, without the automation upgrade. With the automation upgrade installed it works from any side, as you described.

(I hadn't tried the automation upgrade all by itself. It also doesn't go into the machine upgrade slots, so no worries there.)

Resolution:
The canning machine accepts cells and cans from the back, and other ingredients from the top.
With an automation upgrade, any side can be used to import/export items, so add that to make the machine more flexible.

(It is a little confusing to think that most other machines don't need this kind of special treatment.)

@AtomicWaffle14 it was added for the Apply and dont think. Just to get things running. The advanced upgrades are a lot more complex and allow a lot more.