IBM Series 1 User Manual page 87

Qni1Load ,ggui valences. Uni·t load substitutions may be .ade
on the I/O channel using the following equivalence
definitions and rules.
A 'y!!it Qravailable droE.§ is an available resource of
I/O drops (sockets)
(1)
outboard of a processor, or (2)
between the processor and the first channel repower feature,
or (3) between any two repower features, or (4) outboard of
a channel repover feature.
In some cases, a unit of available drops spans two
physical units, as with systems utilizing the 4955 processor
(without battery backup) with a 4959 attached. In general,
a channel repower feature arbitrarily plugged into any I/O
slot defines a unit of available drops. Note that a channel
repower feature, required for a 4953 as a prerequisite for
I/O expansion, defines a unit of available drops within the
4953 processor unit.
In a given unit of available drops, the total load is:
«#
general unit loads) +2 (#TTL unit loads».
This total must be less than or equal to the minimum of
either 21, or the number
of I/O drops in that unit of
available drops subject to the constrant that the maximum
number of TTL unit loads is 8.
For purposes of computation, the unit of available I/O
drops outside of a channel repower feature when no outboard
I/O expansion unit is attached is considered to be
14.
A special case of a unit load sUbstitution is for an
attachment that presents a TTL unit load in all respects,
except that any of its type
A-A, A-B, or A-C rece1vers
always presents an active receiver load (-1.6 mA at 0.4V).
This type of load is called a TTL selected load. A maximum
of one TTL selected load attachment can be substituted for
one general unit load attachment in any unit of available
drops. No TTL unit loads may coexist in a unit of available
drops containing a TTL selected load.
Qth~!: At!.~£hm~!
Considerations
~tiQ!!
Qf Physical
~nd Loq ic
a1
~!~l!~!!1.2.
Each device
attachment must have the capability to redrive or propagate
a poll as part of the serial polling mechanism. This
polling mechanism must be contained on that part of the
device attachment card(s) that plugs into the I/O socket.
This is to ensure that:
1)
cabling delays are not
encountered, i.e. cabling out to the poll mechanism at the
device may cause incorrect timing sequences to occur
depending on cable length and driving/receiving capability,
2) powering down a device must not affect the ability to
propagate a poll.
All I/O channel drivers, receivers, and logic necessary
to condition receivers should be on that part of device
attachment card(s) that plugs into the I/O socket. This
would include at least device address comparison logic, some
2-72
GA34-0033
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