IBM 1 Series - Page 57

Processor-Initiated IPL
When designing the IPL mechanism for an I/O attachment device, there
are several important items that must be taken into consideration. These
items can be categorized into two groups: IPL selection and system reset.
IPL selection refers to the ability of an IPL device to be selected as either
being the primary or the alternate loading device on the channel. Because
only the two IPL selected sources (plus host IPL function) are allowed on
the I/O channel, the processor selects the appropriate device via status bus
bits 0 and 1. The device attachment must have the capability of being
personalized as either primary or alternate or neither. Refer to Figure
2-19. Note that the primary or alternate selection is provided via jumpers.
The processor-initiated IPL sequence consists of two ‘system resets,’ the
second of which performs a sequential timing function. With the activation
of the first ‘system reset’ and ‘initiate IPL,’ a dc reset occurs. This dc reset
resets the IPL tag flip latch and deactivates the ‘IPL’ tag from any
previous IPL request, which may have been in-process or in a hung state.
When ‘system reset’ deactivates, the flip latch is set and the ‘IPL’ tag is
activated. However, before the flip latch changes state, due to delays, a
logical O is set into the enable IPL D-trigger. Therefore, IPL cycle-steal
requests and transfers are not yet enabled. The second ‘system reset’ pulse
becomes active, but does not affect the IPL tag latch because the ‘initiate
IPL’ tag is inactive. The second ‘system reset’ then deactivates and it
causes the active value of the ‘IPL’ tag to be set in the enable IPL trigger,
thus enabling IPL requests and transfers. If a third ‘system reset’ occurs,
the IPL tag and enable IPL latches are reset.
Device Reset
For most normal applications, the recommended implementation to
respond to a Device Reset command (0110 1111; see I/O command
description in a processor description manual) 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.
Receiver Conditioning
Receivers on bidirectional buses may have to be conditioned, depending on
the technology utilized. This conditioning reduces loading on the particular
bus to the source that is driving the bus at the time. Receiver conditioning,
by itself, serves no purpose as an enabling or logical function. Receiver
conditioning is discussed in “‘Receiver Conditioning” under
‘“Driver/Receiver Information” in this chapter.
2-46 GA34-0033