IBM Series 1 User Manual page 67

P['ocesso~Initia~g
Ill:.
When designing the IPL mechanism.
for an 1/0 attachment there are several important items that
must be taken into consideration. These items can be
categorized into tvo groups: 1) IPL selection and, 2) system
reset.
IPL selection refers to the ability of an IPL device to
be selected as either being a primary or alternate loading
device. Since only tvo IPL sources are allowed on the 1/0
channel, the processor selects the appropriate device via
the status bus bits 0 and 1. The device attachment must
have the capability of being personalized as either being
primary or alternate or neither. Refer to Figure 2-19,
Processor initiated 1PL logic. Note that primary or
alternate selection is provided via jumpers. The processor
initiated IPL sequence basically consists of two system
resets, the second of which performs a unique function.
With the activation of the first system reset and initiate
IPL, a dc reset occurs resetting the 1PL tag flip-latch and
deactivating the IPL tag. When system reset deactivates,
the flip latch is set and the IPL tag is activated. But
prior to the flip latch changing state, due to delays, a
logical zero is clocked into the D-trigger. Therefore, IPL
request and transfers are not yet enabled. The second
system reset pulse becomes active but does not affect the
flip latch because the initiate IPL tag is inactive. The
second system reset then deactivates causing the active
value of the 1Pt tag to be clocked through the trigger, thus
enabling IPL requests and transfers. Should a third system
reset occur, the IPL tag and enable IPL latches are reset.
Qevi~ .B~.§~l.
For most normal applications, the recommended
implementation to execute a device reset is to utilize the
entire envelope of address gate as a long strobe. This
allows for a greater length of effective reset and for
earlier clearing of logic.
Re£~i.!~!:
hondi!ioni,!!g. Receivers on bidirectional busses
may have to be conditioned, depending upon the technology
utilized. The primary reason for this conditioning is to
reduce loading on the particular bus to the source that is
driving the bus at the time. Receive.r conditioning
by
itself serves no purpose as an enabling or logical function.
Therefore, receiver conditioning is discussed in more detail
in the subsequent section of this chapter on "Electrical
Characteristics", specifically in "Unit Load
Characteristics".
2-52 GA34-0033
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