Hp-Ib Interface (Option 002) - HP 7475A Service Manual

Section VI
Communication Information Adapter (ACIA) U13, and 2
multiplexers U12 and U27. See Figure 6-2 and 6-24.
6-9. DATA INPUT AND OUTPUT CIRCUITS. Be-
cause signals within the 7475A are positive-true TTL
levels, inverting line drivers and receivers, U9, Ull, and
U26, are used to convert the logic and voltage between
the levels required by the interface system (shown in
Table 6-1) and the internal levels. Figure 6-2 is a diagram
of the 1/0 circuits and the ACIA.
6-10. ACIA. The logic level on the Read/Write input con-
trols the direction of data flow through the data bus
interface. When the RS (Register Select) input is high,
the R/W input selects either the receive or transmit data
register. When RS is low, the control/status registers are
selected. The 1 MHz signal at the E input clocks data in
and out of the ACIA on the data bus (DO-D7) and the
RX/TX clock (Baud Rate Clock) clocks data in and out
on the RXD (Received Data) and TXD (Transmitted
Data) lines,
6-11. MULTIPLEXER Ul2. When the address line (EIOO)
is high, the 7475A transmitted data goes out on the
Transmitted Data line (pin 2). The Secondary Transmitted
Data line (pin 14) is held at a negative voltage. When
EIOO is low, information on the Secondary Received
Data line is retransmitted on the Transmitted Data line
(pin 2), and the 7475A transmitted data goes out on the
Secondary Transmitted Data line (pin 14).
6-12. MULTIPLEXER U27. When the address line (EI03)
is low and chip select (CS) from U6, gate array B, is low,
three control lines are passed onto the data bus. The
functions of these control lines are as follows:
a. DATA SET READY (DSR) - Activated by the
modem to tell the plotter that the modem is
operational.
b. CLEAR TO SEND (CTS) - Activated by the
modem to tell the plotter that it is ready to receive
and re-transmit data from the terminal.
c. DATA TERMINAL READY (DTR) - Activated
by the plotter to tell the modem that the terminal
is operational.
6-13. BAUD RATE. The internal baud rate (transmit/
receive) clock signal is derived in U5, Gate Array A,
from the 4 MHz clock signal. The baud rate clock fre-
quency is sixteen times the desired baud rate. Baud rate
Model 7475A
may be selected by means of the rear-panel switches, or
an external clock signal may be used.
6-14. HP-IB INTERFACE (Option 002)
6-15. HP-IB interface connections are shown on the
schematic diagram at the end of this section. Input/
Output voltage levels must meet TTL requirements (low
= -0.8 V; high= -2.0 V). All signals are active low
(true). Figure 6-3 is a block diagram of the HP-IB Inter-
face Circuits.
6-16. All data and control signals to and from the HP-IB
go through UlO and Ull, the bus transceivers, which
are enabled to transmit or to receive by U12, the inter-
face adapter in response to HP-IB commands. The inter-
face adapter automatically handles all handshake
requirements on the HP-IB. When the plotter power is
on, the bus transceiver inputs are high impedance to
prevent loading the data lines. When the plotter power is
off, the bus transceivers present an open circuit
to
all
data lines; consequently, the plotter does not interfere
with other HP-IB operations.
6-17. A 16 line bus is used to carry data and control
information and is divided into three sets of lines.
a. Data bus - 8 signal lines - DIOl through DI08.
b. Data transfer control - 3 signal lines
(Handshake).
c. Interface management - 5 signal lines.
6-18. The data bus transfers 8-bit data or control words
between the controller and the plotter. The words are in
bit parallel byte serial form. The words are transferred
bi-directionally and asynchronously.
6-19. The three data transfer control lines, or "hand-
shake lines" are used to control the transfer of informa-
tion on the data bus. These lines are identified as follows:
a. DATA VALID (DAV)- Used to indicate that valid
information is available on the data lines.
b. NOT READY FOR DATA (NRFD) - Used to
indicate the readiness of the Plotter to accept
information.
c. Nm' DATA ACCEfYI'ED (NDAC) - Used to indi-
cate the acceptance of information by the Plotter.
Table 6-1. RS-232-C/CCI'IT V.24 Interface Signal Levels
INTERCHANGE VOLTAGE
NOTATION
NEGATIVE POSITIVE
(More Negative than -3 V) (More Positive than +3 V)
Binary State 1 0
Signal Condition
Marking Spacing
Function off on
6-2