Most scanners work in "keyboard emulation mode". That means the data scanned is input into the computer just as though a very speedy typist entered the data for you. (That's why check out clerks can enter the barcode number manually when a code doesn't scan.)
In FileMaker you can either put the cursor in a text field, scan the barcode, then click a button to process the scanned information (check something in or out), or you can use script triggers to automate the process so that all your clerk does is select a mode (Check in or check out) and start scanning bar codes.
If you have access to the necessary technical information, you will probably find that you can program the scanner to precede the scanned data from the barcode with one or more characters and to then also append an additional character such as return/tab/enter to the end of the scanned data. Filemaker will perceive all of this data as though it were typed from the keyboard.
You can select an unusual key combination as the data to put before the scan and use the OnLayoutKeystroke script trigger (Found in Layout setup...), to perform a script that puts the cursor into the text field for receiving the scanned data. This way, you simply scan the data and it automatically appears in the text field.
On a system here where I work, we use this script for that purpose:
If [ Get ( TriggerKeystroke ) = "~" and Get ( TriggerModifierKeys ) = 3 /* Modifier 3 means shift and capslock are down */ ]
Go To Field [MyTable::gBarCodeField]
Exit Script [Result: False ]
The scanner precedes the scanned data with Shift-CapsLock-~ which is a very unlikely key combination to press by accident to trigger this script and put the cursor into the field.
This same field has an OnObjectExit trigger set on it so the appended Enter key we selected to use exits this field and triggers a script to respond to the scan and process the data that was scanned into this field. It also means that we can type in the scanned data and press enter to get the same response.
This is not the only approach. Other approaches are possible, but it works for us.