The recommended unit is the HP HBSW8300 Smart Wand .
Although old, It is still available in some places
A number of other units are known to work Including
The Zebex unit has been superseded, but i believe the later Z3220 should also work, and has an RS232 option, but this has not been tested by Voltech
Whichever Barcode reader unit is used must support the 19200 Baud rate used by the AT3600
Note that the ATi uses a slower 9600 Baud rate , so readers are not necessarily directly transferable unless they can have the Baud rate reset by user , usually by reading a special bar code in their specific Barcode user manual
The Barcode settings for the HP device are published in the ATi user manual .. section 8.3.1 page 8-25/ 8-26
you can use pretty much any barcode format supported by the reader.
this includes (but not limited to)
Code 39,& Extended Code 39
Interleaved 2 of 5,
Essentially, the bar code reader does the conversion from graphic code to ascii , and then gives the AT3600 the result.
There's useful resource on the subject on a number of web pages.