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3-4 Data Exchange
System control in a PC Link System is decentralized; the polling unit merely
handles communications among the PC Link Units. A link is established be-
tween the polling unit and a polled unit when the polled unit confirms a sec-
tion of the LR area allocated to it as a write area.
The polling unit polls each PC Link Unit in the System or Subsystem in turn,
sending the most recent LR data for the other PCs in the System and receiv-
ing the most recent LR data from the PC Link Unit being polled. Data is held
in a PC Link Unit in a buffer that is updated by the PC during the PC Link
servicing portion of the PC’s scan. When Units are polled, the RUN and Error
flag statuses for each Unit in the System are also updated. In the following
diagram T
n
is the transmission time per PC (see below).
Polling unit
Unit 1
Unit 2 Unit n
Polled unit Polled unit Polled unit
T
n
T
n
T
n
Total polling time = T
n
x number of PC Link Units + 10 ms
If an error occurs during transfer, communication is broken either partially or
completely (see 7-1 SR Area Flags for details), preventing LR data from be-
ing updated, although the most recent data is normally preserved, even for
power failures.
The maximum time required per PC scan for PC Link Unit transmissions de-
pends on the number of PC Link Units in the System and the number of LR
bits transferred per PC, i.e., the maximum time is the time required to service
all PCs and update all LR words in each. The equation for this is given below.
The final 10 ms is required for processing at the end of transmission.
Maximum transmission time =
transmission time per PC Link Unit x number of PC Link Units + 10 ms
The minimum transmission time is the transmission time for one PC Link Unit
plus the post-transfer processing time:
Minimum transmission time =
transmission time for one PC Link Unit + 10 ms
Transmission Time per PC Link Unit
Number of LR bits 32 64 128 256 512
Transmission time 2.2 ms 2.5 ms 2.8 ms 3.8 ms 5.8 ms
After 256 cycles of PC Link Unit polling, an induction sequence is executed if
any PC Link Units have not been established in communications. This se-
quence is executed for each non-active PC Link Unit to established commu-
nications and requires 15 ms per PC Link Unit.
Maximum and Minimum
Polling Time
Induction Sequence
Data Exchange Section 3-4