Tandy 1000 SL Technical Reference Manual page 254

Resend
-
FE Hex
AT and
XT
Modes
The keyboard issues a Resend (FE Hex) in response to inputs
which have parity errors, framing errors, or invalid data
received fran the host system.
KEYEOARD
BUFFER
OVERFUN
-
AT and XT Modes
When the 16-character keyboard buffer receives the 17th
character, an overflow condition occurs. This condition is
ccmmnicated to the host system by transmitting the Keyboard
buffer Overrun (FF Hex for XT Mode, 00 Hex for AT M e ) to the
host system.
Prior to sending CQmnands to the keyboard, the host system must
first check to see if the keyboard is sending data. If the
keyboard is transmitting, and the data is past the parity bit,
the host system must accept the data prior to initiating its own
transmission.
If the keyboard's data has not yet reached the tenth clock pulse
(Parity Bit), or is not transmitting data, the host system
a s s m s control by lowering the clock line for a minimum of 60
microseconds, then releasing the clock line after clanping the
data line low to indicate a start bit, The keyboard will respond
with an RTS within 5 microseconds by clocking the start bit into
the keyboard. The keyboard continues to clock data as shown in
the timing diagram (Figure 3B)
.
The host
system
must ensure that
the data is valid prior to the rising edge and after the falling
edge of the keyboard clock pulse.
After the parity bit, the host system should raise the data line
to indicate a stop bit. The keyboard checks for a logical high
stop bit, then clanps the data line low prior to clock in the
stop bit, This signals the host system that the keyboard
received
the
data correctly (Acknawledge). If the host system
has not raised the data line to indicate a stop bit, a
framing
error results and the keyboard continues to clock data until the
data line is raised by the host system. Upon receiving either a
framing or parity error, the keyboard issues a
RESEM)
to the
host system.
All
cxmnands
fran the host system require a response franthe
keyboard. The keybard will respond to these comnands within 20
microseconds.
15