Table of Contents
Advertisement
1ch
It
., as
trol
the
:)ck,
il
is
lost,
.ost.
ling
lock
1ires
that
,e to
smit
llntil
with
I
the
bit
3
.en a
uses
.cters
'eady
send
s, the
ways
ready
cta to
size. )
That
n the
wi
Software Usage and Examples
HP-75 to send an appropriate prompt character to the external device via the modem. For example, the
HP-75 can check the receive buffer for a carriage return while it is reading in data with t'10D HU, and
then send the prompt when it is through processing the line.
If the modem is using Prompt Mode (bit 3 of
K~
set), the prompt character is defined by
K1.
The modem
stops sending data to the host whenever it sends a carriage return. If it finds a
~1
character, and
~.q
is not
the same as
K15,
the modem will also strip off any
K~
characters received between the carriage return and
the
K1
character. If the modem is not set to use a prompt character (bit 3 of
K~
clear), then it doesn't
wait for a prompt before sending data and doesn't stop sending data after a carriage return. Prompt Mode
has higher priority than Transmitter XON/XOFF Protocol.
COMBINED PROTOCOLS. Some devices use both ENQ/ ACK and XON/XOFF protocol. This enables
them to initiate the transfer of data according to ENQ/ ACK protocol, and to stop and start the transfer as
required by the receiver.
When the HP 82718A modem uses ENQ/ ACK and XON/XOFF protocols together, it operates according
to both of these protocols as described above, with one exception.
Prompt Mode and Transmitter
XON/XOFF Protocols cannot be used simultaneously because they both use the
K~
and
K$
characters.
Therefore, if the modem is using Transmitter XON/XOFF Protocol, Prompt Mode should be disabled-if
both protocols are enabled, Prompt Mode has priority.
Buffer Control
The modem monitors the number of data bytes in its receive buffer to determine when to halt
transmission from the remote device by sending the appropriate software handshake character. By not
allowing the external device to send data until there is enough room in the buffer for the next block of
data, the modem can prevent the loss of data.
For ENQ/ ACK protocol, the host normally sends data to the terminal in blocks followed by an
;~Q.
The
modem's receive buffer can hold up to 64 bytes. If the modem is using Receiver ENQ/ ACK Protocol, the
modem will not send an
~~~
until t' 1 OD HU: empties the receive buffer.
If
the modem is using Transmitter ENQ/ ACK Protocol, then the external device will
be
expecting to
receive data in blocks that it can handle. If the external device is expecting a block of data of 40
characters, for example, then the modem will send its data in blocks of 40 characters, each followed by an
ENGl.
The modem will not send out another block until it receives an
AcK
The block size used by the modem with Transmitter ENQ/ ACK Protocol is 8 to 64 bytes, in 8-byte
increments. The block size is specified in bits 2-0 of
~~:
Refer to the owner's manual of the external
device to determine the block size required.
For XON/XOFF protocol, the device that is sending data will continue to send data until it receives an
~Qfr.
The device may send a block of data before it can actually stop sending. Thus it is important to
reserve a block of empty bytes in the receive buffer that is large enough to accept the maximum block
that the external device may continue sending upon receiving an
XPf.f.
If the receive buffer overflows,
received data is ignored until the number of bytes in the receive buffer drops below 56. Then the buffer
overflow response
(OV)
is placed in the buffer, and receiving continues.
When the modem is using Receiver XON/XOFF Protocol, it counts the number of bytes in its receive
buffer. When it has received a number of bytes equal to the block size, the modem sends an
~.or-jt:
(actually, a
J,Sj
character). An ~q~ (actually, a K~ character) is sent after t'10D I t'~$ empties the receive
buffer.
33
Advertisement
Table of Contents
