Table of Contents
Advertisement
The 2265/2845 display system operating in a remote en-
vironment communicates with the IBM System/360. The
display system operates in half-duplex mode, through an
IBM 2701 Data Adapter Unit connected for full-duplex
operation and through appropriate data sets, over common-
carrier facilities (see Figure 1). A choice of two data set
adapters permits data transmission at the rate of 1200 or
2400 bps.
USASCII CODE SET
The United States of America Standard Code for Informa-
tion Interchange (USASCII) is the basic code set used for
all communication exchanges between the System/360
I/O channel and the remote display system. The 2845 is
compatible with the 2848 and, therefore, uses USASCII-
1963 as does the 2848. The current version of USASCII,
USASCII-1967, differs slightly from USASCII-1963; the
graphics shown in positions EO, FC, and FE in Figure 4
are located differently in USASCII-1967. All references to
USASCII in this manual refer to USASCII-1963. Since
USASCII is a seven-bit code and all data transmitted or .
received over the System/360 I/O channel must be of
eight-bit structure, the USASCII code must he modified
to an eight-bit structure. This modified version is called
USASCII-8.
USASCII-8 is used within the I/O channel and for data
transfer between the I/O channel and the 2701. Data
transmission between the 2701 and 2845 is in seven-bit
USASCII code. The 2701 converts USASCII-8 code to
USASCII code for transmission to the 2845. Conversely,
the 2701 converts USASCII to USASCII-8 for transfer to
the I/O channel. Because conversion is automatic, the pro-
grammer need be concerned with the USASCII-8 code set
only. The USASCII characters used in data interchanges
between the System/360 I/O channel and the 2845 Display
Control are shown in Table 11 (hexadecimal code conver-
sion chart) and in Figure 4.
TRANSMISSION CODE BYTE STRUCTURE
Although a knowledge of the byte structure of each trans-
mitted character is not necessary for 2845 programming
and operation, Figures 4 and 5 may prove helpful in under-
standing byte conversion and transmission. Figure 4 shows
the byte structure (bits) of USASCII-8 coded characters.
USASCII bytes are identical except that bit 6 is dropped.
Figure 5 shows the fOlmat of a character (byte) as it is
transmitted over the communications channel (common-
carrier facility). The bits are transmitted serially, the start
bit being first. The start and stop bits perform control
Principles of Operation
functions; the VRC (vertical redundancy check) bit estab-
lishes even parity and is used for error checking only. (See
"Parity Checking".) These three bits are not part of the
USASCII character.
BLOCK DIAGRAM ANALYSIS
As an aid to understanding the operatIon of the remote
2265/2845 display system, the function of the 2701 Data
Adapter Unit and the data sets shown in Figure 1 are
described below.
IBM 2701 Data Adapter Unit
The IBM 2701 Data Adapter Unit equipped with the IBM
Terminal Adapter Type III serves as a data buffer, data
converter, and synchronizer. The 2701 also provides the
System/360 I/O channel with 2845 Display Control status
and sense information through the 2701 status and sense
bytes.
Data transferred from the I/O channel and intended for
the 2845 Display Control is received by the 2701 Data
Adapter Unit in data bytes of USASCII-8 code structure.
The 2701 converts the USASCII-8 data bytes to USASCII
code ( seven bits) and places each byte in a transmit adapter
(XA) register for transmission to the 2845. The 2701
transfers the seven bits of the USASCII character serially
one bit at a time via appropriate data sets and communica-
tions channels to the 2845. The seven bits are preceded by
a start bit and followed by a VRC bit (for parity checking)
and a stop hit. The ten-bit format assumed by every char-
acter transmitted between the 2701 Data Adapter Unit and
the 2845 is illustrated in Figure 5. The start, VRC, and
stop bits are developed by the 2701 and accompany the
seven bits of every USASCII character transmitted to the
2845 in the order shown. The data received by the 2701
from the 2845 is also in the same ten-bit format. All ten
bits are the same signal length and transmitted at the
speed (1200 or 2400 bps) specified by the data set adapter
and data sets used.
Note: For additional information pertaining to the IBM
2701 Data Adapter Unit, refer to IBM Component
Description, IBM 2701 Data Adapter Unit, Form
GA22-6864.
Data Sets
Data sets (sometimes identified as modems, a contraction
of modulator-demodulator) are devices used at each ter-
minal of the communications channel. They provide the
necessary compatibility between the 2701 Data Adapter
Unit, 2845 Display Control, and the communications
Principles of Operation
25
Advertisement
Table of Contents
