IBM 1 Series - Page 124

The basic sequences on Figure 4-6 start from point A with a normal
receive operation, and end with an attention interrupt being posted. Line 1
depicts interrupt acceptance and reading of the receive buffer register
within time T,. IA & R1 reads the character received in RCV OP 1,
ITA & R2 reads the character received in RCV OP 2, and IA & R3 reads the
character received in RCV OP 3.
Line 2 depicts delay in interrupt accept and read beyond T,. In this case,
the character received in RCV OP 2 was lost and the data read by
IA & RI was the character received in RCV OP 1.
Line 3 depicts a delay of greater than two T, times in taking an IA&R
for RCV OP 3. In this case, the characters received in RCV OPS 2 and 3
were lost, and the data from RCV OP 1 was read by IA& RI.
Lines 4, 5, and 6 depict what occurs if the read is omitted from the
interrupt accept. Lines 4 and 5 depict what occurs if an interrupt accept
without read is taken after a condition code 6 (attention and exception) is
posted. Note that the characters resulting from RCV OPS 2, 3, 4, and 5
were lost, and that the character read by IA & R4 was the character
received in RCV OP 1.
Line 6 depicts what occurs if an interrupt accept without read is taken
after a condition code 4 (attention) is taken. Note that the character
resulting from RCV OP 2 was lost. IA & R2 read the character resulting
from RCV OP 1.
If T, has been exceeded, no overrun occurs unless the first stop-bit time
of another character is detected by the attachment before the IA&R
occurs to clear the interrupt and read the first character; line 7 is an
example of this. Even though IA & R1 did not occur until after T,, no
overrun occurred because the first character was read by IA & R1 before
the leading edge of the stop-bit time of the next character.
Unbuffered devices whose input is from the keyboard can generally
tolerate a much longer interrupt mask time than the value of T, listed in
Figure 4-6 because it is the device character rate that determines how long
an interrupt mask time can be tolerated. An unbuffered device may be
operating at 9600 bps, but the character rate is determined by how rapidly
an operator can press keys.
If an operator is entering data on an unbuffered terminal that is configured
for 9600 bps, the Dit rate of any given character is 9600 bps, but the
character rate is probably 15 characters per second or less. The
instantaneous maximum character rate that can be caused by a person
keying in data is 30 characters per second.
For a device to be considered unbuffered, it must be determined that there
is no possible sequence in which the device could transmit two or more
consecutive characters at an instantaneous character rate that exceeds 30
characters per second.
Any device that sends a multiple character burst of data for the pressing of
any single key (for example, “‘here is’’ key) must be considered a buffered
device. Any device that has a multiple byte status that can be read by the
program must be considered buffered.
Teletypewriter Adapter Feature 4-19