Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 379)

Chapter 27
The Status File
27–11
Address Description 5/02
5/01,
Fixed
S:2/15 DH–485 Communications Servicing Selection Bit
Read/write. When set, only one communication request/command
will be serviced per END, TND, REF, or SVC. When clear, all
serviceable incoming or outgoing communication requests
/commands will be serviced per END, TND, REF, or SVC. When
clear, your communications throughput will increase. However, your
scan time will increase if several communication commands/requests
are received in the same scan.
One communication request/command consists of either a DH–485
incoming command, DH–485 message reply, or DH–485 outgoing
message command. See S:2/5, S:2/6, S:2/7.
To program this feature, use the EDT_DAT function to set/clear this
bit. To provide protection from inadvertent data monitor alteration of
your selection, program an unconditional OTL instruction at address
S:2/15 to ensure one request/command operation, or program an
unconditional OTU instruction at address S:2/15 to ensure multiple
request/command operation. Alternately, your program may change
the state of this bit using ladder logic if your application requires
dynamic selection of this function.
Application example: Suppose you have a system consisting of a
SLC 5/02 processor, an APS programmer, and a DTAM. The
program scan time for your user program is extremely long. Because
of this, the programming device or DTAM takes an unusually long
time to update its screen. You can improve this update time by
clearing S:2/15.
In a case such as this, the additional time spent by the processor to
service all communications at the end of the scan is insignificant
compared to the time it takes to complete one scan. You could
increase communication throughput even further by using an SVC
instruction. See chapter 18.
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