Rs-232C Interface - Radio Shack TRS-80 Service Manual

RS-232C INTERFACE

The RS-232C interface utilizes a 4-pin DIN con-
nector (J2). This interface allows the computer
to have serial communication with printers,
modem, or other computers. The four signals
used by the interface are:
1. CD — a status input line
2. RS232lN — serial data input
3. GROUND — zero voltage reference
4. RS232OUT — serial data out
The pin configuration for the DIN connector is
shown in Figure 3.
4 1
RS232 OUT
Figure 3. RS-232C Connector Pin-Out
In general, an RS232C signal uses negative
logic. Therefore, a voltage greater than +3 volts
is defined as a SPACE, or logical 0. A voltage
less than -3 volts is defined as a MARK, or logi-
cal 1. The range of -3 to +3 volts is undefined.
The RS-232C interface circuitry is shown on the
upper right corner of the schematic. The output
signal from the CPU output P20 is tied to a
741C op-amp (U16). This same output is also
used for the cassette output, so care must be
taken to ensure that a cassette output does not
appear as an RS-232 output. The op-amp is ref-
erenced at 1.4 volts by resistors R2 and R3.
This reference causes the op-amp to swing be-
tween the two power supply voltages (Vcc and
Vee) as the TTL input switches states. A 100
ohm resistor (R1) is included to provide a cur-
rent limit on the output. The two input signals
(RS232lN and CD) utilize identical circuits and
share a common bias
RS232IN (pin 4 of U15) is tied to CPU input P22
and CD (pin 6 of U15) is tied to CPU input P23.
The inputs from the external device are con-
nected to the positive side of a diode. This
diode blocks the application of a negative volt-
age to the comparator (U15). When a positive
voltage is applied, the diode conducts and the
voltage is applied to the input. The compara-
tors are referenced at two volts, by R5 and R6,
CD
3
RS232 IN
2
resistor network.
thus if the input voltage is greater than 2.6
volts, the comparator is turned on. The compar-
ator outputs are open-collector so pull-up
resistors R8 and R9 are required.
[I think there is a consistent error in the documen-
tation of the pins here, specifically, CD and
RS232IN are reversed.]
1. RS232lN — serial data input
2. CD — a status input line
3. GROUND — zero voltage reference
4. RS232OUT — serial data out
[And the diagram should look like:]
RS232 OUT
[and the text should read...]
(RS232lN and CD) utilize identical circuits and
share a common bias resistor network. CD (pin 4
of U15) is tied to CPU input P22 and RS232IN (pin
6 of U15) is tied to CPU input P23."]
– 14 –
4 1
RS232 IN
3
CD
2
"The two input signals