PC Serial Port Receiver
This circuit was designed to control a 32 channel Christmas light
show from the PC serial port. Originally designed with TTL logic,
it has been simplified using CMOS circuits to reduce component
count. It is a fairly simple, reliable circuit that requires only
4 common CMOS chips (for 8 outputs), an optical isolator, and a
few discrete components. The schematic diagram (SERIAL.GIF)
illustrates the circuit with 16 outputs which can be expanded with
additional 8 bit shift registers.
Disclaimer
This circuit requires physical connections be made to the
computer's serial port (COM1 or 2). To the best of my knowledge,
it is difficult to cause damage to yourself or your computer
by improper connections to this port, but there is no guarantee
that damage will not result. Use caution when making any external
electrical connections.
Basic RS232 serial transmission
Serial data is transmitted from the PC as a series of positive and
negative voltages on a single wire which occur at predetermined
times established by the baud rate. Both the transmitter and
receiver must be operating at the same baud rate so that the
receiver knows when to expect the next bit of information. For the
PC serial port, baud rate and bit rate are the same thing, but this
is not necessarily true with modems that can detect more than two
states of the line.
In the quiescent state, with no load on the line, the voltage on
the transmit line (pin 2 of the 25 pin connector) will be about
-12 relative to the signal ground (pin 7), which corresponds to a
logical "1". The output impedance of the serial port is about 1K
ohm which yields about 6 milliamps at 6 volts. A typical data
transmission frame consists of a start bit, 8 data bits, and one
to three stop bits. The start bit which is always positive,
signals the beginning of the transmission and is used by the
receiver to synchronize the clock so that the data bits can be
sampled at the proper times. After the 9th time interval passes
(start bit plus 8 data bits) a dead time occurs which allows the
receiver time to get ready for the next character. This dead time
is referred to as a stop bit, which is always negative or the same
as the quiescent state. The circuit described here requires two
stop bits of dead time for reliable operation. More sophisticated
circuitry would require only one.
Transmitted character examples
The letter "A" has a ASCII decimal value of 65. The "1" and "64"
bits are transmitted as a negative voltage (logical "1"), and the
others are transmitted as a positive voltage (logical "0").
64 + 1 = 65 = "A"
+ _____ _____________________________ _____
| | | | | |
| | | | | |
- ----- ----- ----- ---------
Start D0 D1 D2 D3 D4 D5 D6 D7 Stop Stop
Decimal value 1 2 4 8 16 32 64 128
Receiver's
Clock _______ __ __ __ __ __ __ __ ____________
| | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | |
-- -- -- -- -- -- -- --
The letter "B" has a ASCII decimal value of 66. The "2" and "64"
bits are transmitted as a negative voltage (logical "1"), and the
others are transmitted as a positive voltage (logical "0").
64 + 2 = 66 = "B"
+ ___________ _______________________ _____
| | | | | |
| | | | | |
- ----- ----- ----- ---------
Start D0 D1 D2 D3 D4 D5 D6 D7 Stop Stop
Circuit operation
The input terminals (pins 1 and 2) of the optical isolator are
connected through a 1K resistor to the transmit and signal ground
pins of the PC's serial port (pins 2 and 7 of the 25 pin connector).
A small signal diode is connected across the isolator input
terminals to protect the isolator from reverse voltage. In the
idle state, the isolator input voltage will be about -0.7 volts
and the isolator LED and transistor will be off. When a start bit
is received, about 5 milliamps will flow through the isolator LED
causing the isolator transistor to conduct at about 80 microamps
which in turn causes the external switching transistor (Q1) to
turn off. The rising voltage at the collector of Q1 is coupled
through a 510 pF capacitor to produce a narrow positive pulse
which sets the Q output of the first RS data latch (1/2 CD4013)
and enables the dual NAND gate clock oscillator.
The clock oscillator runs at a frequency equal to the baud rate
(9600 Hz) and must maintain a frequency accuracy of less than 5%
over the temperature range. High stability R and C components
are recommended.
The clock output is delayed by one cycle so that the start bit
will not be received as a valid data bit. This is accomplished
by the two remaining NAND gates (1/2 CD4093) and the second RS
data latch (1/2 CD4013). One of these gates is used to invert
the clock phase so that the first clock edge seen by the latch
(clock pin 11) will be going the wrong direction and so ignored.
The remaining gate, which is enabled by the second latch, opens on
the third clock edge, but also inverts the clock phase, and so
supplies a falling clock edge to the counter and shift registers
which is again the wrong direction, and is ignored. The fourth
clock edge will be rising and active and will occur near the
middle (about 52 microseconds) of the first data bit which will be
shifted into the registers. The remaining 7 bits are shifted into
the registers on each successive rising clock edge. Data is
inverted at the register outputs, a logical "1" will correspond to
zero volts, and a logical "0" will correspond to +6 volts.
Transmitting character (255) will set all outputs low, and
transmitting character (0) will set them all high.
The 4017 decade counter increments one count on each rising clock
edge and resets both data latches on the 8th edge. This in turn
stops the clock and resets the counter, and the circuit remains in
a waiting state until the next start bit arrives. Two stop bits of
dead time are required to allow the voltage at the input of the
NAND gate (pin 2) to reach a logic "1" before the next start bit
arrives. Erratic operation may occur when 2 or more characters are
transmitted as a string and only one stop bit is used.
The circuit may be modified to run at different baud rates by
adjusting the clock frequency. This can be accomplished by
temporally connecting pin 6 of the CD4013 to the positive supply
and then selecting R and C values for the desired frequency. You
may need to use a 1% resistor or a couple 5% resistors in series
or parallel to get the value close enough. Or use a variable
resistor in series of about 10% the total value.
At 9600 baud, data output at the shift registers will be unstable
for about a millisecond per word while the incoming data bits
are shifted into the registers and the existing bits are shifted
out (into bit heaven). Higher baud rates will reduce this time
proportionally and the original circuit operates at 57.6K baud
to eliminate a slight flickering of the lights which was noticed
at 9600.
The 74HCT164 shift register outputs will sink or source about
4 milliamps at 6 volts which can be increased with medium power
transistors or FETs to drive relay coils, incandescent lights
and other electronic devices. If relays are used, a small signal
diode will need to be added across the relay coil to suppress
the inductive voltage.
Power supply
It is recommended that 0.1 uF capacitors be installed near the
power pins of each CMOS device and a well regulated/filtered power
supply be used. For test purposes, a 6 volt battery will work
but the clock frequency will change slightly with power supply
voltage variations.
CD4011 Quad NAND gate
14 | Vdd
________|_______
| |
| CD4011 |
| |____ |
1 -----|----| \ |
| | 0 ---|----- 3
2 -----|----|____/ |
| | |
| |
| |____ |
5 -----|----| \ |
| | 0 ---|----- 4
6 -----|----|____/ |
| | |
| |
| |____ |
8 -----|----| \ |
| | 0 ---|----- 10
9 -----|----|____/ |
| | |
| |
| |____ |
12 -----|----| \ |
| | 0 ---|----- 11
13 -----|----|____/ |
| | |
|________________|
|
7 | Vss
CD4013 Dual 'D' Type Flip-Flop
14 | Vdd
________|_______
| |
6 -----| Set 1 Q1 |-----1
5 -----| D1 |
3 -----| Clock 1 _ |
4 -----| Reset 1 Q1 |-----2
| |
| CD4013 |
| |
8 -----| Set 2 Q2 |-----13
9 -----| D2 |
11 -----| Clock 2 _ |
10 -----| Reset 2 Q2 |-----12
|________________|
|
7 | Vss
CD4017 Decade Counter/Divider
16 | Vdd
________|_______
| |
| CD4017 |
| |
| "0" |----- 3
| "1" |----- 2
| "2" |----- 4
14 -----| Clock "3" |----- 7
| "4" |----- 10
13 -----| Clock "5" |----- 1
| Enable "6" |----- 5
| "7" |----- 6
15 -----| Reset "8" |----- 9
| "9" |----- 11
| Carry out |----- 12
|________________|
|
8 | Vss
74HCT164 8 Bit Serial-In / Parallel-Out Shift Register
14 | Vdd
________|_______
| |
| 74HCT164 |
| |
1 -----| AND Gated Q0 |----- 3
| Serial Q1 |----- 4
2 -----| Inputs Q2 |----- 5
| Q3 |----- 6
| Q4 |----- 10
9 ----0| Reset Q5 |----- 11
| Active Q6 |----- 12
| Low Q7 |----- 13
| |
8 -----| Clock |
|________________|
|
7 | Vss
Serial port male D-SUB connectors as seen from outside the PC.
1 13 1 5
_____________________________ _____________
( . . . . . . . . . . . . . ) ( . . . . . )
\ . . . . . . . . . . . . / \ . . . . /
------------------------- ---------
14 25 6 9
Name Output/Input 25 pin 9 pin
---------------------------------------------------------
Transmit Data O 2 3
Receive Data I 3 2
Request To Send O 4 7
Clear To Send I 5 8
Data Terminal Ready O 20 4
Data Set Ready I 6 6
Ring Indicator I 22 9
Data Carrier Detect I 8 1
Signal ground - 7 5
Power line ground - 1 -
QBasic test program for 8 bit receiver
CLS
DEFINT A-Z
PRINT "Test sequence in progress, press any key to quit."
OPEN "COM1:9600,n,8,2,CD0,CS0,DS0,OP0,RS,TB2048" FOR OUTPUT AS #1
Sequence:
FOR Bit = 0 TO 7
PRINT #1, CHR$(255 - (2 ^ Bit)); ' Set one of 8 outputs high.
SLEEP 1 ' Wait 1 sec between characters.
IF INKEY$ <> "" THEN CLOSE : SYSTEM
NEXT Bit
GOTO Sequence
END
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PC Serial Receiver (57.6K Baud / TTL & CMOS)
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