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The normal sequence for buffer write operations
begins, after selection of a 2250, with a Set Buffer
Address Register and Stop control command followed
by a Write Buffer command. Command chaining can
be used. The Set Buffer Address Register and Stop
command stops regeneration and sets the buffer
addressing circuitry for the selected 2250 to the ad-
dress at which writing is to start. The data bytes
following the Write Buffer command are stored in
consecutive buffer locations. ill the write opera-
tion, data transmission is terminated under chan-
nel byte count control. Once the data is written,
display generation can be started by a Set Buffer
Address Register and Start command.
If
the chan-
nel attempts to write past the last buffer location,
wrap-around occurs (writing continues from the
first buffer location); this destroys any data previ-
ously stored in these locations. Writing into a
buffer location that contains a cursor causes the
cursor to be removed.
All data bytes are parity checked as they are
received from the interface. Detection of a data
parity error does not terminate the write opera-
tion; however, it does cause the Unit Check bit to
be set in the status byte. The Unit Check indication
is sent to the channel as part of ending status when
the write operation is completed. A data parity
error also causes the Bus Out Check bit to be set
in the sense byte. No parity correction is per-
formed on data bytes with parity errors. In re-
sponse to the unit check status, the program should
issue a Sense command for further definition of the
check condition. Then, after analyzing the sense
data, the CPU should correct the data
in
the buffer
by rewriting. Since the 2840 does not perform par-
ity correction on the data byte(s) with bad parity,
an attempt by the program to start regeneration
without correcting the data in the buffer will result
in a buffer-parity-initiated interrupt.
Read Commands
Read commands initiate information transfer from
the 2840 or from the selected 2250, via the 2840,
to the channel at a maximum rate of
1.
9
µs
per
byte. Several types of Read commands can be
used, depending on the optional features installed
in the 2250 (Table 8). Any Read command bit
configuration that is not valid for the selected 2250
causes the setting of the Unit Check bit in the status
byte and the Command Reject bit in the sense byte.
Read Buffer Command
This command causes the transfer of sequential
buffer data bytes to the channel via the
I/
0
inter-
face. Buffer regeneration for the selected 2250
34
must be stopped for this command to operate;
failure to stop regeneration causes the Unit Check
bit (in the status byte) and the Command Reject and
Buffer Running bits (in the sense byte) to be set.
The Set Buffer Address Register and Stop con-
trol command stops regeneration for the selected
2250 and specifies the first buffer location to be
read. Command chaining can be used. Once buf-
fer regeneration is stopped, single or multiple
read operations can be performed. The read oper-
ation is terminated by channel byte count control,
which determines that the number of bytes speci-
fied by the program have been read,
If
the chan-
nel attempts to read past the last buffer location,
wrap-around occurs (reading continues from the
first buffer location). Note that whenever a loca-
tion that contains a cursor is read, only the data,
not the cursor, is sent to the channel.
All bytes are checked for correct parity as they
are read from the buffer. Detection of a parity
error does not terminate the read operation (parity
is corrected on data being sent to the channel), but
does cause the Unit Check bit to be set in the status
byte. This status is sent to the channel when the
read operation is completed, A parity error also
causes to the Data Check bit to be set in the sense
byte. ill response to the Unit Check status, the
program should issue a Sense command for further
definition of the check condition. A Set Buffer
Address Register and Start command is used to
continue regeneration for the selected 2250 after
the Read Buffer command is completed.
Read Manual illput Command
This command is used to transfer alphameric key-
board or programmed function keyboard information
to the channel. Activation of the alphameric key-
board END or CANCEL key or any programmed
function key causes the Attention bit to be set in the
status byte when the 2250 is interrogated and if the
status and sense registers are clear, The 2840
interrogates the 2250 when regeneration is in pro-
gress, When regeneration is not in progress, the
regeneration timer is automatically restarted when
the time period expires. In this case, interrogation
of the 2250 for attention status occurs each time
the timer is automatically restarted, The attention
status is passed to the channel at the earliest ap-
propriate time, such as at the end of current chan-
nel operations, causing an I/
0
interrupt.
If
the
2840 cannot pass this status to the channel before
the next 2250 is interrogated, the status of the
next 2250 is not accepted by the 2840 until the 2840
status and sense registers have been cleared by the
2840 - channel interface operations. When a key
is activated, the keyboard remains locked until
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