There should be a call to the cash drawers API. Read the technical manual of the hardware you are using. Thanks.

Use this app: https://github.com/Fedrojesa/ProxyPoS it's created for work as a driver and make your PoS module on OpenERP can communicate directly with local ECS/PoS compatible devices.

Let me know if you have any question or need help.

There are receipt printers with a link to the cashdrawer. When you print the receipt the drawer will automaticly open.

Update: In the updated POS in version 8 there will be more support for ticket printers, barcoe scanners and cashdrawers.

Most printers will spit out a configuration sheet if you turn it off, hold down the form feed button, and then turn the printer on. It will tell you things like parity, XOn/XOff, bit rate, etc. Open up the advanced tab of your printer properties and make sure the setting match the config sheet. Now some peoples are using Partner Tech 5E-415 Cash Drawer which are now available on stores as Discount Till Rolls Ltd.