Note: Descriptions are shown in the official language in which they were submitted.
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FLIP STATUS LINE
Background of the Invention
This invention relakes generally to fault checking, and
more particularly to fault checking of slave devices.
It is known to have a master device control several
slave devices. This may be done by the use of buses. In such a case,
each slave device will have a unique address associated with it. A
poll address bus is used to address each slave device in turn, and a
poll mode bus is used to send control signals ko the addressed slave
device to elicit a particular response.
For example, the poll address bus may have seven lines
which means that up to 128 slave devices may be involved. One test
that is desirable to be able to perform is to test for the presence or
absence of a slave device; another is to test for the operational
status (i.e. operating or not operating) of a slave device that is
present.
The test may be accomplished by using two bits to reply
to the test. For example, the logic bits 11 returned by the slave
device may indicate that the slave device is present and operational
while the response 00 may indicate that the device is present but not
operational. However, a fault may exist on the line drivers, causing
the drivers to be stuck in the logic 1 position. Using known
techniques this is difficult, if not impossible, to detect.
Summary of the Invention
The present invention is directed to a method and a
circuit for detecting such faults. Briefly stated, the present
s~
invention operates by polling, in turn, all the device locations.
Each reply is noted and, after one complete polling cycle, the master
control then asserts a "flip status line" which causes each slave
device being polled to return the inverse of its reply signal. In
other words, the device that replied with a logic 11 on the first
polling cycle will now replay with a logic 00 iF its status has not
changed and if it is working properly. If it replies with a logic 11
(i.e. the same reply on both the first and second polling cycle) a
fault condition exists.
Sta-ted in other terms, the present invention is a
method of verifying the response to a poll, of a device that is polled
periodically so as to report its current status to a central control
circuit, the method characterized by: polling the device periodically
and asserting a control signal for every otiler poll so as to cause the
device to invert the bits in its response signal when the control
signal is asserted whereby the non-inversion of the bits during the
period of several polls is indicative of either a fault condition or a
device not present condition.
Brief Description of the Drawings
The invention will now be described in more detail wi-th
reference to the accompanying drawings, wherein like parts in each of
the several figures are identified by the same reference character,
and wherein:
Figure 1 is a simplified block diagram depicting one
embodiment of the present invention;
Figure 2 is a chart containing signal values useful
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for understanding the present invention.
Figure 1 depic-ts a simplified block diagram of master
control circuit 20 connected to slave devices 21a, 21b, 21c, and 21d
referred to collectively as slave devices 21. Master control circui-t
20 is connected to slave devices 21 by the following busesO Poll
address bus 22 is a seven-line bus used to send address signals to
slave devices 21. Data bus 23 is a bidirectional eleven-line bus for
the transfer of data between master control circuit 20 and slave
devices 21. Poll mode bus 24 (four lines) is used to elicit response
from slave devices 21; that is, poll address bus 22 specifies the
address of the slave device 21 which is to respond, and poll mode bus
24 poses the question to be answered~ Note that poll mode bus 24
sends three types of signals. They are as follows: poll-no
message; poll-messagej and poll-allocate (i.e. which slave device 21
has next use of data bus 23).
Status bus 26 is a two line bus used to return the
status of a slave device 21 to master control circuit 20, in response
to a poll on poll address bus 22 and poll mode bus 24. The final line
of Figure 1 is flip status line 27.
Flip status line 27 is used to cause slave devices 21
to return the inverse of their normal response signal whenever line 27
is asserted (i.e. logic 1).
This is shown graphically in Figure 2. The first four
lines of the chart in Figure 2 represent the response of a slave
device 21 with Flip status line 27 not asserted (i.e. logic 0). The
responses are 00 representing "Down" (i.e. devîce present but not
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operating); 10 represen-ting "Write" (i.e. device wishes to write or
transmit data on data bus 13); 01 representing "Read" (i.e. device
wishes to read or receive data from data bus 13); and 11 representing
"Up" (i.e. device is present and operational but wishes no use of data
bus 13).
The last four lines of the chart of Figure 2 represent
the responses of a slave device 21 with flip status line 27 asserted
(i.e. logic 1). This causes slave devices 21 to invert the bits in
their status reply and "Down" now becomes 11, "Write" becomes 01,
"Read" becomes 10, and "Up" becomes 00.
In operation, master control circuit 20 polls slave
devices 21a, 21b, 21c, and 21d; it then keeps repeating that polling
sequence over and over again. Every other polling cycle, by master
control circuit 209 has flip status line 27 set at logic 1. If the
status of slave device 21 has not changed from one polling cycle to
the next, then the response of slave device 21 should alternate
bet~een a given signal and its inverse from one poll to the next.
For example, assume that slave device 21a is "Up" and
consequently on its first poll (flip status line 27 has a logic 0) it
responds on status bus 26 with a logic 11 signal. On the second poll
of slave device 21a (flip status line 27 has a logic 1) and device 21a
responds on status bus 26 with a logic 00 signal (assuming no faults
and assuming no change of status). If device 21a were to respond to
the second poll with a logic 11 signal, this would indicate a fault
(assuming no change of status).
Note that if a device is not present, the reply on
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status bus 26 will be a logic 00 regardless of the sta-te of flip
status line 27. In other words, status bus 26 will alternate between
"Down" and "Up" which indicates slave device 21 is not present.
If a slave device 21 is present, but not operative
(e.g. it has just been added to the system and is undergoing self
test) it will reply to a poll (no message) with "Down" (logic 00).
On the next poll it will respond again with "Down" (but this time with
logic 11, due to flip status line 27). Thus, master control circuit
20 can distinguish between a connected, but inoperative slave device
21 and a missing slave device 21.
This scheme can also detect errors in the drivers of
slave devices 21 that drive status bus 26.
