A Supermarket Data Communications Network

Introduction

A mere decade ago it would have seemed strange to discuss the problem of data communications for the supermarket. However, as store management followed the digital technology revolution there has been an increasing need for data communications in the supermarket. Of course, on more than a cursory look this may seem obvious. Besides the checkout counter with both cash registers and bar code readers, digital devices, used for measurement can be found at scales, thermometers, barometers, security devices and on a host of other platforms. The trend has been to take the digital outputs of these devices and collect them at a centralized store computer. Applications can then be run on the computer to support inventory control and a variety of management decisions.

This data collection task poses several problems. First, the digital devices are generally too far from the computer to allow reliable communication by just a simple cable connection. Secondly, the devices are often placed in a "harsh" environment, subject to severe variations in temperature, humidity, spillage and other deleterious effects- almost the reverse of a "clean room." This drives the solution to the data collection task to be a data communications network designed for operation in such an environment.

In this application note a description is given of the data communications network design problem which was faced by an actual supermarket chain in the western United States. The data communication needs and the constraints placed on network implementation are presented. A general description is given of the network which resulted. We then concentrate on one subnetwork element of the total network. Implementation of this subnetwork demanded a special network device. This brought the supermarket data communications network planners to Telebyte Technology, Inc. and resulted in the development of Telebyte's Model 290 RS-422 / RS-485 Concentrator - Wiring Hub.

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The Requirements

The supermarket of interest was a chain of 325 stores located on the Pacific coast. While data communications traffic requirements differed somewhat from store to store, the difference was essentially in magnitude. The same types of digital data devices had to be linked consequently, the same type of network had to be implemented.

The types of store devices which had data communications needs are enumerated below:

All of these devices had to be connected in a relatively harsh environment. A great part of each store is cleaned each night with hot steam hosing. As a result, the presence of device failure due to spillage is quite high. Distances over which to be communicated ranged from several 10's of feet to approximately 1,000 feet.

The Network Design

As one would expect the in-store local data communications networks had been developed and installed in order to satisfy the above constantly changing requirements. The network designers were directed to develop a unified architecture making use of the, sundry, existing data communications networks as subnetworks. Furthermore, they were constrained to use the data communications interfaces on existing store data devices. Much of this equipment had not previously been tied into a data communications network. While interfaces existed on many devices these generally related to data communication facility technologies older than LANs. The end result of satisfying these constraints was a unified network architecture. However, this unified network architecture was essentially a "patchwork" of existing networks supplemented by some additional data communication technology which "sewed" the seams of the patchwork, i.e., together provided the "glue."

What was the Resulting Network Architecture?

A Series I computer, operating under SNA, was connected by a leased line to the offsite host computer. The PC's were connected in an IBM token ring. The other store devices (checkstand scales, scanning cash registers etc.) were connected to their respective controlling PC's by either EIA-422 or EIA-485 type networks. These individual "PC network layers" were driven in design by the types of interfaces at the end user devices.

The Outside Service Department Scale Subnetworks

In the design of the unified network for each supermarket special attention was given to the subnetwork which connected the service department scales and the PC which was the platform for the product database. Why did this merit special attention? The service departments are comprised of the fish, meat, deli and bakery departments. In the past, the only requirement with respect to the service department scales was to be able to tell the customer the correct weight and price of an item. This could be done effectively within each department without any communication to elsewhere in the store. However, state government regulations, governing the supermarket chain, changed this situation. Regulations required that a customer be given the actual ingredients and content breakdown of the purchased items. This information had to be retrieved from the product database. This subnetwork had to satisfy this requirement. Some of the scales had EIA-422 interfaces and others had EIA-485 interfaces. Consequently, the subnetwork had to be able to deal with either. Distances between the PC which was platform to the product data base and the individual service department scales were in the range of 200 to 300 feet.

Figure 1: Supermarket Service Department Subnetwork

Given these requirements the supermarket network architects came to Telebyte Technology, Inc. for assistance. Telebyte Technology, Inc. responded by developing a new product, the Model 290 EIA-422/EIA-485 Concentrator- Wiring Hub. This allows the PC to communicate with a service department scale regardless of whether or not it has the EIA-422 interface or the EIA-485 interface. The Model 290 allows the service department subnetwork illustrated in Figure 1 to be implemented. It has 16 ports and interfaces to the PC through the EIA-232 interface which is the "master" port. This master port supports Transmit Data, Receive Data and Ground. The Model 290 then incorporates interface conversion to both EIA-422 and EIA-485 so that communication can be effected to service department scales. Each scale is tied into a "slave" port of the Model 290 which is set by dip switch to be either EIA-422 or EIA-485. The Model 290 is pictured in Figure 2.

Figure 2: Model 290 - Telebyte's Model 290 Wiring HUB

Because the Model 290 is so unique it will be worthwhile to summarize some of its interesting features. While this unit does serve the older EIA-422 and EIA-485 technologies its special features give networks based upon these standards added flexibility. As a result, they assist in just those network design situations as encountered here, where existing equipments having these interfaces must be dealt with.

The ports of the Model 290 are all surge protected and isolation is provided between the ports. Internal intelligence allows the Model 290 to support half duplex operation on the EIA-485 single twisted wire pair without the requirement for Request To Send (RTS) on the master port. Full duplex operation is supported on the EIA-422 ports over 2 twisted pairs. In a conventional EIA-485 network a single node failure can crash an entire network. However, the Model 290 isolates any single failure. Conventional EIA-485 networks are also limited to 32 nodes. The Model 290 design breaks this constraint allowing up to 496 modes. Multiple Model 290's can be cascaded together to allow EIA-485 networks in excess of 496 nodes.

EIA-485 In the Supermarket Environment

The Telebyte Technology, Inc. Model 290 was developed because one particular supermarket chain had a problem with its scales with the EIA-485 interface. This really was no accident. While this particular supermarket chain had not previously made use of the EIA-485 many stores have and continue to make use of networks based upon this interface. Basically, the standard provides high speed data interchange for multidrop networks of up to 32 nodes when using a single twisted pair cable. As such, it is a convenient way to implement a polling network. Many store network architects have found that such a multidrop network was attractive for linking a store's main computer with its cash register stands, scales and other data devices. Consequently, the Model 290 has a broad application in the store environment and is not limited to the specific example above for which it was developed. In fact, it is finding use in many applications where data communications is effected by a multidrop polling network. Figure 3 provides an illustration of its use one such typical network which may be in a store or an industrial plant or elsewhere.

Figure 3: A typical Network with Model 290

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