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					                                Warszawa, 1.06.2005




  EMISY Project



Cash desk controller
   Supervisor PhD T. Starecki
1) Introduction

POS states for point of sale and is a set of various devices useful for a clerk working with a
cash register. The main part of POS set is cash register by itself. It controls and contains:


      barcode scanner
      receipt printer
      cash shelf
      keyboard
      LCD or LED display
      Touchscreen monitor
      Magnetic card reader
      Fast Ethernet connection for new codes from a weight in grocery section


In my implementation because of the economy factor that is crucial I did not consider
touchscreen monitors, magnetic card readers nor microcontrollers supporting fast Ethernet
connection such as MPC855. Our market allows us to choose a cash register out of almost one
hundred types but none of them cost less than 1500PLN – simple cash register that does not
include barcode scanner. In this specification it is shown that there is a possibility of a great
discount form this price that bounds the market from below. The discount means possibility of
selling designed here device.


2) Design

A block diagram of designed cash desk controller looks as follows:
AT25256A 256Kbit
SPI EEPROM                                                                      LCD


                                                                                LCD


                              Microcontroller
                              AT89S8252                                 16 keys keyboard




                            RS-232C interface




                              Barcode reader                              Printer




 3) How does it work?

 INPUT                             FROM THE                       OUTPUT
                                   INSIDE
 Barcode scanner reads barcode Microcontroller process            Processed price is delivered to
 and transports it in the binary   received data, searches for    the receipt printer in a binary
 form via RS232C interface to      a relation “barcode-price”     form via RS232C interface and
 the cash register. If a barcode   in additional EEPROM           appears on both LCD (one for
 is unreadable for the scanner,    memory connected to            a customer and one for a
 a clerk has to type it form the   microcontroller via SPI        clerk). Two lines and 16 chars
 keyboard.                         interface. Once it finds the   LCDs allow displaying both
                                   relation, price is fetched     sum and current product price.
                                   and processed.
4) Implementation and detailed description of the components

  a) Microcontroller -


  the core of the system which main function is to make selections from the database stored
  in additional serial EEPROM memory. Database in details will be explained later. I chose
  AT89S8252 by ATMEL microcontroller with internal 8Kbytes of Flash memory of a code
  which when it comes to databases can be spacious. The additional 2KB of EEPROM
  memory allows for storing and logging day and month profits. The XTAL1 and XTAL2
  pins are connected by 12MHz crystal resonator, which allows for a sufficient speed of
  serial interfaces.




  b) LCD


  Classic type of LCD 2x16 display, in fact two of them connected in parallel to P2,
  working only in write mode (R/W pin grounded). 2 lines of the module allow to split
  display into two parts SUM: XXXX.XX and current price PRODUCT_CODE:
  XXXX.XX. In the algorithm that controls LEDs there need to be a software
  implementation of a delay to allow displays working properly in write only mode.




  c) Keyboard


  Matrix keyboard of a size 4x4 connected to eight pins of P0 with additional resistors since
  external pull-ups are required. Matrix keyboard contains 16 keys and implemented by
  software functional keys. First shift_lock must be pressed and released to get into
  functional mode, now all the digits works as functional keys, to get back into regular
  mode shift_lock must be pressed and released again.
d) Barcode scanner


Barcode scanner is connected to the system via RS232C interface with MAX232A voltage
converter (RS232<-->TTL/CMOS). Barcode scanner is a very expensive device but a
store without barcode scanner will serve customers for 10 minutes each and soon become
empty so the idea of buying it is worth considering. Barcode scanner is an option. In this
design barcode scanner is connected to COM 1(TXD, RXD pins of port 3) which means it
is hardware implementation of RS232. I chose barcode scanner CD-2800R.




e) Receipt printer


Receipt printer is connected to the system in the same way as barcode scanner, using the
same MAX232A voltage converter but the implementation of its communication with
microprocessor is done by software. Receipt printer is the crucial part of the system since
it logs all the operations performed on a cash register. It also prints receipt for customers.
Receipt printer is the most expensive device in this design. I chose EPSON receipt printer.




f) MAX232A


MAX232A is a simple converter (driver) of the RS232 signals to TTL/CMOS compatible
and in the other way. MAX232A allows converting signals for two RS232 interfaces.




g) Serial EEPROM 256Kbit SPI memory by ATMEL


ATMEL AT25256A is connected to the 51 microprocessor with SPI interface. It is the
main memory of the systems as it contains data base of 13 digit integer of barcode and a
corresponding 6 digit integer of price where two last digits state for tenth parts of PLN
namely GROSZE. One relation with additional information (some products are often
purchased than others) needs approximately 8 Bytes. Making a research in my local store I
found out that there is about 2000 products. So the memory needed for this store is about
   16Kbytes. This memory allows creating database for 4000 products. System memory can
   be easily extended to 64Kbytes or even 128Kbytes using the same memory chips and
   connecting them in the following way:




5) Prices

Atmel 89S8252                                 5$
LCD 2x16 HMC16225                             3,5$
LCD 2x16 HMC16225                             3,5$
Receipt printer EPSON TM-U220                 222,5$
Barcode scanner CD-2800R                      100$
MAX232A                                       3$
Serial EEPROM AT25256A                        2$
12MHz crystal resonator                       0,9$
Resistors and capacitors                      0,5$

Total                                         340,9$
6) Conclusion


The price shown above cannot be a final price of sale since implementation of software will
be very laborious work and the price will reach lower boundary of the market 1500PLN. We
need to consider that it indeed costs 1500PLN but it include barcode scanner. Without
barcode scanner the total final price will be approximately 1200PLN. Software
implementation allows us to program microcontroller in the way that it has many useful
features. Also extending serial EEPROM memory (cannot be even considered as an expense -
2$ for 32KB) gives us possibility to sell this design to very large shops.

				
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posted:4/25/2013
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