Commands - IBM 2250 3 Manual

The specific commands, their codes, and the
minimum features required for their use are listed
in Table 8. The coding is in hexadecimal, which is
shown in Appendix A. In addition to the four basic
types of commands, the 2840 responds to the Test
I/O and Halt I/O CPU instructions. The commands
accepted as valid by the 2840 depend on the features
attached to the selected 2250. When a command that
refers to an optional feature is presented and the
selected 2250 does not have that feature, the com-
mand is treated as invalid and is rejected, causing
no 2840/2250 operation.
Table 8. Commands Used with the 2840-2
Feature Requirements*
Programmed
Channel 2250
Command Alphameric Function
Command Command Code
Keyboard Keyboard
Write
Write Buffer
01
Read Read Buffer
02
Read Manual Input OE
x
(or)
x
Read Cursor 06
x
Read XY Position
Registers 12
Read Output
Registers**
OA
Control No Operation 03
Set Buffer Address
Register and Start 27
Set Buffer Address
Register and Stop 07
Insert Cursor
OF
x
Remove Cursor 1F
x
Set Programmed
Function Indicators 1B
x
Set Audible Alarm OB
Sense Sense 04
*X - The special feature required for operation of the command.
Blank - The special features that do not affect operation of the
command.
**Read Output Registers is a diagnostic command which is accepted
and performed only when the 2840 CE key switch is in the CE po-
sition and the CHECK STOP switch is in the DEVICE STOP position.
The command causes four 2840 registers to be read out.
When loading the buffer, bytes are normally
received from the channel in the following sequence:
1. A unit address byte, which selects the 2840
and a 2250. This selection is accomplished in
the 2840 interface control section.
2. A Set Buffer Address Register and Stop com-
mand, which stops regeneration for the
selected 2250
(if
in progress) and initiates
a request to the channel for two address
bytes. When received, these two address
bytes are passed by the interface control
section and the buffer register to the ad-
dress register associated with the selected
2250, selecting a buffer starting address.
3. A Write Buffer command, which specifies
that a buffer write operation is to be per-
formed and initiates a request to the channel
for data bytes. When recei".ed, these data
bytes are written into consecutive buffer
locations, starting at the location specified
by the address register.
4. If operating with an alphameric keyboard,
a Set Buffer Address Register and Stop com-
mand, which selects the address for insertion,
can be followed by an Insert Cursor command,
which causes a cursor to be inserted.
Display generation for a 2250 is initiated in the
following sequence:
1. A unit address byte is received from the
channel, selecting the 2840 and the 2250
at which the display is to be generated.
2. A Set Buffer Address and Start command is
received from the channel; this command
initiates a request to the channel for two ad-
dress bytes. When received, the address
bytes are placed in the address register for
the selected 2250, specifying the buffer loca-
tion at which byte retrieval for display gen-
eration will start. The command then allows
the 2840 to read two bytes from the buffer,
thereby initiating display generation.
The above sequence can then be repeated for
another 2250. Display regeneration for all 2250s is
interrupted while display generation is being initi-
ated for any 2250.
Programming Note: Unless bit 34 (the Suppress
Incorrect Length Indication bit) of the Read Buffer
or Write Buffer CCW is a 1, the channel program
will be terminated, even if the CCW Command
Chaining bit is on.
Write Buffer Command
The Write Buffer command causes the data bytes
received by the 2840 following the command to be
placed into consecutive buffer locations at a maxi-
mum rate of 1. 9
µs
per byte (526, 000 bytes per
second) into the buffer. If regeneration is in pro-
gress when the Write Buffer command is received,
the command will not be accepted, the Unit Check
bit will be set in the status byte, and the Command
Reject and Buffer Running bits will be set in the
sense byte.
2840 Operations with the Channel 33