Tektronix 2230, Service Manual

Page: 268 / 483

Options— 2230 Service
ROM chip (U1243). The resultant function is to enable the
ROM whenever COM SEG is LO and either BLKO or BLK1
is LO. This enable drives the output enable pin of U1243
and not its chip enable pin which is driven by A18.
The other half of U1235 provides a similar function for
U1242, the RAM chip. It generates a LO-going strobe
when COM SEG, RAM DIS, and BLK3 are LO and BLKO
and BLK1 are HI. RAM DIS disables U1242 if the non­
volatile RAM is present. Although the RAM has images
throughout the 88000 to 8FFFF address range, only the
highest image is used.
Half of U1232 and inverter U1244C generate the
PATEN signal for the bidirectional data bus buffer U1231.
DATEN is LO for any reference in COM-SEG and for refer­
ences to the option I/O ports. It is LO when DEN, the data
enable from the processor, is LO and either COM SEG or
I/O 20PT (U1245 pin 3) is LO.
RAM and ROM
Temporary storage for the option is provided by RAM
U1242. Option operating system firmware is contained in
ROM U1243.
UART
The UART U1251 communicates with the Microproces­
sor, providing serial-to-parallel conversion and handling
some of the RS232 protocol. Also included is an internal
baud rate generator. Crystal Y1251 provides a time base
which is divided by software selectable ratios to provide
the required bit transfer speeds. Three interrupt lines,
INTR, TBRE, and DR, inform the Microprocessor that
intervention is required.
Line Drivers
Driver U1225 translates from TTL logic levels to the lev­
els required by the EIA RS-232-C standard. It requires
positive and negative supplies which are derived by diodes
isolation (CR1224 and CR1223) on the +8.6 V and
-8 .6 V supplies. Diode isolation protects the instrument
from transients or faults coupled through the RS-232-C
connectors. The RLSD signal is generated by Interrupt
Mask Latch U1236.
The RS-232-C receiver is U1224. It translates from
RS-232-C levels to TTL logic levels and also has a pro­
tected supply. Its + 5 V supply is generated by dropping
the +8.6 V supply through zener diode VR1232. The
IRSLD2 signal goes to Status Buffer U1223.
All of the RS-232-C signals are protected by diode
arrays CR1221 and CR1222, and zener diodes VR1221
through VR1224. Any transients that exceed a ± 2 5 V
range are clipped by the networks.
Two connectors, J1212 and J1214, are provided to
make interfacing easier. The male DB-25 connector con­
forms to the DTE (data terminal equipment) specifications
of RS-232-C, and the female DB-25 connector conforms to
the DCE (data communications equipment) specification.
Only one of the connectors may be used at one time.
Interrupt Circuitry
Two interrupt lines from the UART, INTR and DR, are
combined via OR gate U1234B, generating the DR + INTR
interrupt line. That signal is then routed to U1232A, an
AND-OR-INVERT gate, where it is gated with DR + INTR
MASK, which comes from the Interrupt Mask Latch
(U1236). When DR+INTR MASK is LO, DR + INTR can
not propagate through to the output. TBRE is similarly
masked by TBRE MASK, then they are ORed together
and inverted within the AND-OR-INVERT gate. Inverter
U1244D inverts the signal and applies it to the base of
Q1221. Transistor Q1221 inverts the signal to INTR, driv­
ing the Microprocessor maskable interrupt.
Interrupt Mask Latch
Interrupt Mask Latch U1236 provides four signals that )
are directly controlled by the Microprocessor. It is enabled
when the Microprocessor writes to the addresses decoded
as L4TCH. This latch uses BAO and BA1 to select either
0D, ID, 2D, or 3D, and latches the data present on U1235
pin 13 into the selected output when enabled. Two of the
outputs are used for interrupt masking, one for the RS-
232-C port, and one for diagnostics. The outputs are
forced LO by the BRST line to insure that interrupts are
masked when the Microprocessor powers up.
Parameter Buffer
This circuit is an eight-bit input port for selecting param­
eters associated with the option such as baud rate and
parity. It consists of buffer U1222, switch SI 221, and
resistor pack R1222. The switch is sensed by enabling the
buffer which gates the buffer inputs onto the data bus.
Bit 7 is used to sense serial data out (SDO) from U1251
for diagnostic use.
Status Buffer
Status buffer U1223 is used to sense three positions of
Parameter switch SI 221 as well as miscellaneous other
signals. Functions of the Status buffer are shown in
Table 7-36.
7-42