Rockwell-automation 2708-NBD VBASIC Language Development Kit User Manual download pdf (Page 27)

Chapter 4

Special Devices in A-B VBASIC

4–6

Reserved device names: HOST, NET, and QUE

It is very easy to forward the data collected by a workstation to the host

computer attached to the Master Workstation. We recommend the BASIC

Language Development Kit subroutines SendQue and Send. However, if you

are not using these, the simplest method is to OPEN the “file” named HOST

and WRITE # or PRINT # to it.

Two important features are that:

1. The data is forwarded to the host computer regardless of the network role

played by the unit. Stated differently, output to HOST works the same for

a workstation which is a Master, Concentrator, or any other workstation

type.

2. Once a record has been written to HOST, no further action by the A-B

VBASIC program is required to get the information to the host. If the

workstation is offline, the data is stored temporarily in a FIFO queue and

forwarded when network conditions permit. This occurs transparently to

the application.

Figure 4.1

Data I/O through a Network

Three Special Host Communication Defices

OUTPUT

NET

OUTPUT

TO HOST

OR QUE

INPUT

FROM

QUE

INPUT FROM

HOST OR

NET

NETWORK

OUTPUT

BUFFER

QUE

NETWORK

INPUT

BUFFER

TO HOST VIA THE NETWORK

FROM HOST

This diagram shows the various methods which can be used to move data

into and out of a workstation through its network.

All output operations to HOST enqueue data in a FIFO (first-in-first-out)

queue, which is a special form of RAM file. The network task removes

records one at a time as it is able to transport them to the host computer.

Output operations to the NET device bypass this queue. The record is placed

in an output buffer which is checked before the queue. Use this method

sparingly for high priority alarms which must bypass queued records.

Device: Host Computer

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Rockwell-automation 2708-NBD VBASIC Language Development Kit User Manual Page 27

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