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The remote scanner operates in half-duplex
mode, (two-way, non-simultaneous transmission)
at 1800 bits per second. Two pairs of Type 4B
private phone lines are used with the data sets in
order to eliminate turn-around time in the telephone
lines and equipment.
In
order to conserve line time, during the 20 ms
delay for analog scanning, a digital input group with
an address identical to the last 6 bits of the analog
point address will be automatically transmitted to
the central processor. Thus, the digital input
function operates in two modes: with the analog
input function, or independently.
For every request received by the remote
scanner, the scanner will return one reply, except
that analog input requests may return either one
error replay or an analog reply plus a digital reply.
All output requests cause a non-data reply. All
input requests cause a non-data error reply, or a
data reply
if
the scanner detects no errors.
The remote scanner is controlled by the GE/
PAC computer by transmitting
19
data bits from the
arithmetic unit to the communications coupler,
40
utilizing and OUT or TOM instruction. Bit patterns
are shown in Figure 11.
By transmitting a request with bit #18, the
repeat bit, set at
11
1
11
,
followed approximately 20 ms
later (during which the request, less the repeat bit,
is returned in reply) by a similar request with the
repeat bit set at
11
0
11
,
the remote scanner may be
required to do a complete bit-by-bit word check
before acting on the request.
For maximum reliability, the repeat option
should be used with output requests. Optimum per-
formance with inputs is obtained by doing a complete
check of two successive inputs within the central
processor by program, rather than by using the
hardware repeat function at the remote scanner.
Scan rates may be estimated for short lines
(no repeater stations) by using 20 ms at the trans-
mission time for one word. Thus, one output group
may be energized or one input group read each 40
ms without repeat, or each 80 ms with the repeat
option. In 30 ms, without repeat, one analog point
and one digital group may be read. Overall scan
rates vary with individual systems, depending on
the mix of inputs and outputs, the number of remote
scanners, and the number of voice-grade lines.
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