Rockwell Automation 2708-DH5B2L_DH5B4L Attended Workstation - Manual (Page 102)

Chapter 12
Special Devices in A-B VBASIC
12–6
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 12.1
Data I/O through a Network
Three Special Host Communication Defices
OUTPUT
TO
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