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Data Communications
Receive Errors
When receiving data from the remote device, the terminal
will detect the following three types of error conditions (in
addition to parity errors):
1. Character overruns-a character is received before the
preceding character was processed by the terminal's
data comm firmware.
2. Framing errors-no stop bit was detected at the end ofa
character (this may go undetected for a single-character
transmission or for the final character in a string of
received characters).
3. Buffer overflows-the entire allocated buffer space is
filled (both the working buffer and the overrun area).
The final character received before this condition was
detected will be overwritten. Note that if receive pacing
is enabled and the remote device is using the selected
form of pacing, this condition should never occur.
Receive errors, when detected, are reported to the remote
device by way of byte 5 of the primary terminal status
bytes. The remote device will not be able to determine
which type of error occurred. If multiple receive errors
occur simultaneously, only one will be reported as per the
following order of precedence:
1. Framing error
(highest priority)
2. Parity error
3. Character overrun
4. Buffer overflow
(lowest priority)
Transmit State
In a half duplex environment, the following RS-232C sig-
nals are required for the terminal to be in transmit state:
Request to Send (RS/CA) high
Data Terminal Ready (TRlCD) high
Secondary Request to Send (SCA) low
When filling in either of the half duplex data comm
configuration menus, you may specify whether the ter-
minal is to be in transmit or receive state after its power is
first turned on or after the terminal is reset
(Ini tStl!lt
field). it is important that the terminal and the remote
driver or controller program be in complementary states
when the communications link is being established. Ifboth
are in transmit state or ifboth are in receive state, the link
cannot be established.
Receive State
In a half duplex environment, the following RS-232C sig-
nals are required for the terminal to be in receive state:
Request to Send (RS/CA) low
Data Terminal Ready (TRlCD) high
Secondary Request to Send (SCA) high
When filling in either of the half duplex data comm
configuration menus, you may specify whether the ter-
minal is to be in transmit or receive state after its power is
7-14
first turned on or after it is reset
(Ini tSti!!lt
field). It is
important that the terminal and the remote driver or con-
troller program be in complementary states when the (
communications link is being established. If both are in
transmit state or if both are in receive state, the link
cannot be established.
Local/Remote Modes
The data communications portion of the terminal operates
independently of whether the terminal is in local or
remote. If the terminal is switched from remote to local
while data is being received from the remote device, the
data comm portion of the terminal continues receiving
data (it does NOT halt the transmission). In such a case, the
data received while the terminal is in local is discarded by
the terminal's data comm firmware. The transmit/receive
state of the terminal is not affected by a transition from
remote
to
local or from local to remote.
Full Duplex Operation
In a full duplex environment, the HP 2624A is capable of
transmitting and receiving simultaneously. The ability to
transmit may be inhibited temporarily, but it is never
exclusive of the ability to receive.1\vo physical sets of data
lines are required; control lines are needed only when hard-
ware handshaking or a modem is used. 'fransitions on the
control lines have no effect upon the actual trans-
mit/receive state of the terminal.
When the terminal is connected to the host computer by
way of a modem, the following primary control lines are
required:
Request to Send (RS/CA)
Clear to Send (CS/CB)
Data Terminal Ready (CD/TR)
In addition, the following control lines may be used:
Receiver Ready (RRlCF)
Data Signal Rate Select (SRlCH)
Secondary Request to Send (SRS/SCA)
Secondary Receiver Ready (SRRlSCF)
Half Duplex Operation
In a half duplex environment, the HP 2624A has two
mutually exclusive states: transmit and receive. Although
the transmission line may physically have two sets of data
lines, the terminal functions as though there is only one set
(that is, it assumes that transmissions must be prevented
while the terminal is receiving, and vice versa). A line-
turnaround occurs when the terminal switches from one
state to the other. A half duplex configuration implies that
a modem is present.
The transition from receive state to transmit state, and
vice versa, is governed by the particular half duplex line
protocol being used. In order to complete an attempt to
switch from receive state to transmit state, Clear
to
Send
(CB) must be high. Ifcircuit assurance is enabled, then the
(
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