Rockwell-automation 2708-NBD VBASIC Language Development Kit Manual de usuario descargar pdf (Pagina 29)

Chapter 4

Special Devices in A-B VBASIC

4–8

For example:

PRINT #NETDEV, “BAD INVENTORY NUMBER”

may be trapped by the host computer as a BASIC ERROR because the

complete record sent to the host will look something like this:

0101000100BAD INVENTORY NUMBER

If the host program is testing for zeroes in digits 9 and 10, and is then

looking for “BA” to indicate a BASIC ERROR, it will become confused by

this error message, and potentially abort the programs.

We suggest that, if you must use NET instead of HOST, make your first two

bytes something odd like **.

7. A maximum length is imposed on all records moving through the

network. Consult Appendix D in this manual.

8. AnyA-B VBASIC program reading from HOST or NET must do so at a

rate fast enough to “Keep up” with the host. If a new record arrives before

the previous one is input, the “lost” count is incremented on the #5 status

display.

We highly recommend that you utilize the subroutines contained in the

BASIC Language Development Kit (Catalog No. 2708-NBD) to do all I/O in

the workstation. These library routines allow you to simulate the workstation

on a PC during development, saving you hours of time and headaches. When

using the BASIC Language Development Kit, you will only need to call

OpenLINX, then execute a ReadEvent and wait for a return. Any data

received from the devices (slot reader, COM ports, etc.) will be returned in

appropriate variables, and flags will be set to let you know where the data

came from. Refer to Appendix C for details. If you must use the devices

directly, the following programming examples will show you how:

OPEN “HOST”

Host should be opened only as a sequential or random access file, in keeping

with the record oriented nature of network I/O. QUE may be OPENed for

binary access so long as the general restrictions are observed.

An OPEN “HOST” has a very important side effect: it allows the network

task to deque records from the QUE and forward them to the host.

Program initiation CLOSEs HOST. Program termination leaves HOST

unchanged.

Device: Host Computer

(cont’d)

Details of Specific Statements

and Functions

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Rockwell-automation 2708-NBD VBASIC Language Development Kit Manual de usuario Pagina 29