IBM 1 Series Manual page 22

Halt or MCHK. This is an outboundtag received byall I/O devices. The
tag meansthat either: (1) a Halt I/O command has been issued by the
program or (2) a machine-checkclass interrupt (excluding a storage parity
check) has occurred. Whenthis tag is detected by an I/O device, the
device must disable selection, block poll propagation, and clear anystatus,
states, requests, interface control logic, and registers, with the following
exceptions:
e Residual address
e Prepare level and I-bit
e DI, DO, AI, and AO settings
e Timer values
e Those registers not addressable by the software
e Two-channel switch operation monitors
The receiver for the 'halt or MCHK'tag is always enabled.
System Reset. This is an outbound tag received by all I/O devices. The tag
is singular in nature and meaning. Whenthetag is detected, the I/O
device must disable selection, block poll propagation, and clear any status,
states, requests, registers, and interface control logic, with the following
exceptions:
e Residual address
e DI, DO, AI, and AOsettings
e Timer values
e Those registers not addressable by the software
During IPL sequences, one system reset has a unique function, as
described in ''Processor-Initiated IPL'' under '"'Design Considerations for
Operational Sequences"' in this chapter.
The receiver for this tag is always enabled.
Power On Reset. This is an outbound control line from the power supply
to all system components, and is activated on all power on/off sequences.
While the line is active, all system components are held in a system reset
state. The residual address; DI, DO, AI, and AO settings; timer values;
and operation monitors are also reset. The receiver for this line is always
enabled.
Each processor and I/O expansion unit on the I/O channelhas a
self-contained unit power-on reset. These power-on resets are not
electrically connected among units.
Processor I/O Channel 2-11