Application: Point of Sale System
Quatech Product Used: QSC-200/300 Four Port RS-422/485 Serial PCI Board

Quatech RS-422/485 serial boards are currently being used to implement Point-of-Sale (POS) systems in retail outlets throughout the world. The Quatech board is used as the communication link between a host computer and a network of cash registers and coupon printers. RS-422 adapters were chosen for this application because of the long-distance serial link needed (signals in an RS-422 network can be transmitted over 4,000 feet). In addition, the multi-point communication capabilities of RS-422 were desirable so a single COM line could be used to control 10 separate devices. Using one of Quatech's four port serial boards permits a single card to control up to 40 devices (20 coupon printers and 20 registers)--more than enough for the average store. For simplicity, RS-422 was employed because the individual peripherals needed only to communicate with the master, not with each other. (Had all points in the system needed to talk to each other, then RS-485 would have been the better choice, even though implementing the system would have cost more.)

A typical point-of-sale system is pictured above. The PC contains all the data needed by individual registers and coupon printers throughout a retail location. Quatech PCI asynchronous RS-422/485 serial adapters are installed in PC expansion slots and configured for RS-422 communication. The number of expansion slots available in a system will vary with each PC. RS-422 cables are run between the adapter and the individual serial printers and serial registers. The cashier presses buttons on the register which transmit the item purchased to the host computer through one serial line, the host then returns the appropriate price to the register so it can be displayed for the customer. The host also checks the purchase against a file of potential coupons, and if the purchase qualifies for a special offer, the host sends the printer instructions to print a particular coupon.(A similar system could also be implemented using a serial bar-code reader instead of or in conjunction with a register.)