RS-232 Data Diode
Tutorial and Reference Manual
DRAFT
Douglas W. Jones
University of Iowa
July 28, 2006
Contents
Introduction . . . . . . . . . 1
Use of the RS-232 Data Diode . . . . . . 1
Cables . . . . . . . . . . 2
Signal Definitions for the DB9 RS-232 Interface . . . . 3
Electrical Requirements . . . . . . . . 3
Alternative Cables . . . . . . . . 4
Inside the RS-232 Data Diode . . . . . . . 5
The RS-232 Data Diode Transmitter . . . . . . 6
Transmitter Schematic and Parts List . . . . . . 7
The RS-232 Data Diode Receiver . . . . . . 8
Receiver Schematic and Parts List . . . . . . 9
Intellectual Property Rights – Who owns the RS-232 Data Diode? . 10
The current version of this document and related documents are available from
http://www.cs.uiowa.edu/~jones/voting/diode/
© Copyright 2006, Douglas W. Jones.
This documentation was developed at the University of Iowa with funds from the
National Science Foundation under Grant No. CNS-052431 (ACCURATE) during 2006.
Any opinions expressed here are those of the author and do not reflect the opinions of
either the University of Iowa or the National Science Foundation.
It is the intent of the author to put the final draft of this work under something like the
GNU Free Documentation License: http://www.gnu.org/copyleft/fdl.html
Tom Bowersox actually built and tested the prototype RS-232 Data Diodes. His printed
circuit board design was based on early drafts of this documentation, and the
documentation was revised, when necessary, to reflect the results of his experiments.
Introduction
The RS-232 Data Diode allows one-way data flow from the serial port of one computer
system to the serial port of another. This can be used, for example, to protect a secure
source computer system from potential threats posed by an insecure destination system.
Use of the RS-232 Data Diode
If properly connected, the RS-232 Data Diode should be invisible to the software on the
source and destination computer systems. As with any serial communications, the source
and destination computer systems should be configured to use the same data rate, the
same number of bits per character, and the same parity setting. We recommend the
following for the serial data diode:
Data-rate: We have found that 1200 baud is reliable, and we have used rates up to 9600
baud. Experimentation is justified here, as the maximum baud rate depends, in part,
on the cables you use.
Bits-per-character: This will usually be set to 8, to conform with the needs of most
common character sets including Unicode encoded in UTF-8 format. It can be set to 7
for classic 7-bit ASCII.
Parity: The usual options are mark, space, none, even and odd. Each of these may have
some use. Because the RS-232 Data Diode does not support a reverse channel, and
because opening the RS-232 Data Diode for inspection during use is normal and can
cause transmission errors, we recommend using parity. For 8-bit data transmission,
even and odd parity work equally well; for 7-bit data, odd parity is slightly better.
Because the RS-232 Data Diode has no reverse channel, there is no way for the receiver
to signal that data has not been correctly received. Therefore, the higher level data
should include provisions for forward error correction. The simplest approach to this has
two components: First, each transmitted record should include a checksum of some kind
so that the entire record can be discarded if it contains an error, and second, all records
should be retransmitted many times.
More complex error correcting codes may have some benefit in correcting single-bit
errors, but remember that opening the cover on the data diode may cause signal dropouts
that last for many record-times. Therefore, error correcting codes will not eliminate the
need to retransmit all records periodically.
1
Cables
The RS-232 Data Diode allows one-way data flow from an RS-232 serial port of the
source computer system to an RS-232 serial port of the destination computer system:
Each computer system sees the RS-232 Data Diode as a piece of data communications
equipment (DCE). The serial ports of most computer systems conform to the standards
for data terminal equipment (DTE). Therefore, the RS-232 Data Diode usually uses a
pair of DCE to DTE cables. The most common cable, used with TIA 457 (IBM PC
compatible) 9-pin serial ports, is wired as follows:
Note: All diagrams of cable connectors given here show the pinout as it appears when
looking at the face of the connector from the outside.
This cable (or equivalent) is available from many sources. The connectors are commonly
called DB9 connectors, and this cable is commonly called a straight-through DB9-male to
DB9-female shielded cable.
The cable should be shielded. That is, the metal shells at each end of the cable should be
connected by a shield conductor that encloses the others. This makes it hard to use the
data lines as radio antennas. In addition, the metal shell of the connector on each
computer should touch the computer's grounded metal frame, so that the shield itself does
not serve as a useful antenna.
Another way to prevent the use of the cable's shield as an antenna is to use a cable that
incorporates an integral ferrite bead around the cable at or very near the computer system
end of the cable. Clip-on ferrite beads for this purpose are available.
2
Signal Definitions for the DB9 RS-232 Interface
Pin Number Name Direction Function
1 DCD DTE<DCE Data Carrier Detect – some computer systems require
this to be positive before data transmission.
2 RxD DTE<DCE Receive Data – data received by the computer system.
3 TxD DTE>DCE Transmit Data – data sent by the computer system.
4 DTR DTE>DCE Data Terminal Ready – the computer system should set
this positive when it is ready to receive data.
5 GND Signal Ground – the electrical return path for all
signals; this line should not connect to the cable shield.
6 DSR DTE<DCE Data Set Ready – some computer systems require this
signal to be positive before data transmission.
7 RTS DTE>DCE Request To Send – the computer system should set this
positive prior to attempting to transmit data; the
destination computer system should set this negative.
8 CTS DTE<DCE Clear To Send – some computer systems require this to
be positive before data transmission.
9 RI DTE<DCE Ring Indicator – used only with modems; ignored by
the RS-232 Data Diode and need not be connected.
Electrical Requirements
Before using the RS-232 Data Diode with a computer, it is appropriate to check that the
serial port will work. First, measure the voltage between each pin on the DB9 serial port
connectors and pin 5 (GND). None should be higher than 15 volts (positive or negative).
Pins 3, 4 and 7 (TxD, DTR and RTS) should show at least 7 volts (positive or negative).
A few older computers used more than 15 volts. These may damage the RS-232 Data
Diode. Some cheap computers have used voltages below 7 volts and may not work .
Second, measure the current from pins 3, 4 and 7 (TxD, DTR and RTS) to pin 5 (GND).
If this is over 500 milliamps (positive or negative) the port is not RS-232 compliant. If it
is below 20 milliamps, there may not be enough power for the RS-232 Data Diode.
3
Alternative Cables
If the destination computer system cannot supply sufficient current on pins 3, 4 and 7
(TxD, DTR and RTS), or if it is desired to reduce the number of wires in the cables to
prevent use of the extra wires to transmit covert data, alternative cables can be used.
The alternative source cable has
only two conductors inside the shield,
TxD (pin 3) and GND (pin 5).
In addition, this cable includes
loopback connections inside the shell
of the connector to the source
computer system. The RTS output
from the source computer system (pin
7) is fed back into the CTS input (pin
8), and the DTR output (pin 4) is fed back into the DSR and DCD inputs (pins 1 and 6).
These connections are identical to those established by a standard “loopback connector”
used for serial port testing. A ferrite bead should be used at the computer end.
The alternative destination cable
using AC power has only two
conductors inside the shield, RxD
(pin 2) and GND (pin 5).
This cable includes loopback
connections inside the shell of the
connector to the destination
computer system. Power for the
RS-232 Data Diode is provided
through an auxiliary AC supply, typically a plug-in transformer. This provides power
through TxD (pin 3) and GND (pin 5). Use of this power supply option may limit the
data rate to 1200 baud. A ferrite bead should be used at the computer end.
The alternative destination cable
using DC power is identical to the
AC alternative, except for the power
provisions.
The DC cable includes connections
for a pair of 9-volt DC supplies,
typically 9-volt batteries. These
provide positive power to RTS (pin
7) and negative power to DTR (pin
4), both relative to GND (pin 5). A ferrite bead should be used at the computer end.
4
Inside the RS-232 Data Diode
The cover on the RS-232 Data Diode is designed to be opened in order to allow easy
inspection of the circuitry inside. This allows an outside observer to verify that the
circuitry present performs only the authorized functions. By design, this circuitry avoids
complex and difficult to understand components.
It is safe to open and inspect the RS-232 Data Diode in subdued light while data is being
transmitted. It is also safe to unplug it for inspection and then re-attach it to the cables
after inspection, even if data was being transmitted at the at the time. Obviously,
opening or disconnecting the RS-232 Data Diode may introduce transmission errors, but
all applications designed for use with any data diode must tolerate such errors by design.
The RS-232 Data Diode Consists of two halves, the transmitter and receiver. These are
completely isolated from each other electrically. The transmitter is usually connected to
the source computer system and is powered by it. The receiver is usually connected to
the destination computer system and is powered by it.
The transmitter converts electrical signals from the source computer system to optical
form, using a pair of red LEDs. The receiver uses a pair of phototransistors to convert the
optical data back to electrical form. The top LED/phototransistor pair transmits positive
signals, while the bottom pair transmits negative signals.
The circuit board has wires printed on only one side. Removing it from the case and
holding it up to a bright light should be sufficient to disclose that there are no hidden
circuit traces buried inside the board. In normal use, one or the other of the LEDs should
be on at all times, and when data is being transmitted, both LEDs should appear to be
flickering. When the data line is idle, the bottom LED will be on solidly.
The data rate is limited by the impedance of the driven system (the cable plus the
destination computer system itself). The primary limit on the baud rate is the gain of the
phototransistors in the receiver.
5
The RS-232 Data Diode Transmitter
There are three separate elements to the RS-232 Data Diode transmitter: The transmitter
circuit, the shielding loop, and the loopback connections.
The transmitter circuit connects the TxD signal
(pin 3) to GND, the signal ground (pin 5).
All of the power required to operate the transmitter
is provided through the TxD line. When TxD is high
(from 5 to 15 volts), current flows up through the top
diode and resistor to make the top LED glow; when
TXD is low (from -5 to -15 volts), current flows
through the bottom diode and resistor, making the
bottom LED glow. The resistor limits the current so
that the LEDs will not burn out when connected to a
computer that can drive more than 20 milliamps.
The shielding loop is a conductor that is tied to the
shell of the cable connector, and through the cable
connector to the shield of the cable.
This loop extends the cable shield in order to make
it difficult for conductors inside the shield to act as
antennas to transmit or receive signals from
outside. The most significant potential antennas are
the long vertical conductors between the two LEDs.
Placing the shielding loop adjacent to these
conductors significantly reduces their potential
utility as antennas.
The loopback connections only matter if the cable
to the source computer system includes
connections for the DTR, CTS, RTS, DSR and
DCD signals.
The RTS output from the source computer system
(pin 7) is fed back into the CTS input (pin 8), and
the DTR output (pin 4) is fed back into the DSR
and DCD inputs (pins 1 and 6). These connections
are identical to those established by a standard
“loopback connector” used for serial port testing.
In the alternative cable suggested for connecting the source computer system to the RS-
232 Data Diode, these “loopback” connections are wired inside the cable.
6
Transmitter Schematic and Parts List
LED1 and LED2 – Red LEDs with non-diffusing package with maximum current ratings
of 20 milliamps, a nominal forward voltage drop of 2.2 volts, and a maximum reverse
voltage greater than 3 volts. Panasonic LN28RCPP or LN28CPP should work. The non
-diffusing package lowers the minimum operating voltage while making alignment
between the transmitter's LEDs and the receiver's photodetectors somewhat critical.
R1 – 680 ohm ¼ watt resistor. For a maximum operating voltage of 15 volts and a
maximum current of 20 milliamps through the LED, we use Ohm's law, with a current
of .020 amps and 15 volts minus the forward voltage drop of the LEDs, 2.2 volts. This
gives R1 = (15-2.2)/.020 = 640 ohms. We round this up to the nearest standard 5%
resistor value, giving us 680 ohms. The power is (15-2.2)×.020 = 0.256 watts.
The RF screen for the transmitter consists of just the shielding loop around the transmitter
circuit. It is not connected to the receiver or to the shielded enclosure. This not only
prevents the creation of a ground loop, but it maintains complete electrical isolation
between the transmitter and receiver.
7
The RS-232 Data Diode Receiver
There are four primary elements to the RS-232 Data Diode Receiver: The power supply,
the receiver circuit, the shielding loop, and the loopback connections.
The power supply circuit takes power from
RTS, TxD and/or DTR (pins 7, 3 and 4).
So long as one of these pins goes negative
sufficiently frequently, the negative supply
capacitors will remain charged between –7
and –15 volts. So long as one of these goes
positive sufficiently frequently, the positive
supply capacitors will remain charged
between +7 and +15 volts. This allows for
AC or DC power or for power from the
destination computer system.
The receiver circuit uses the power supply
to deliver a signal to RxD (pin 2).
When the top phototransistor is illuminated,
it connects the positive supply to RxD
through a resistor. When the bottom
phototransistor is illuminated, it connects the
negative supply to RxD through a resistor.
The resistors prevent damage if both
phototransistors are illuminated, for example,
when the case is open so that room light falls
on both phototransistors at once.
The shielding loop operates identically to that in the transmitter, except that it is also in
electrical contact with the screws attaching the RS-232 Data Diode to its case.
The loopback connections use the power
supply to provide signals to DCD, DSR and
CTS (pins 1, 6 and 8).
These signals are all set positive, indicating
that the RS-232 Data Diode is operating.
In the alternative cables suggested for
connecting the RS-232 Data Diode to the
destination computer system, “loopback”
connections are wired inside the cable.
8
Receiver Schematic and Parts List
PT1 and PT2 – NPN phototransistors; PNA1801LS, able to handle a maximum forward
current of 20 milliamps and a forward voltage of 15 volts or more, with a collector
current of at least 3mA at 500 Lux, peak sensitivity near 800nm, and a 4µs response time.
Redesign with a transistor amplifier on each phototransistor should increase the data rate.
R1, R2 – 680 ohm ¼ watt resistors. To limit the phototransistor current to 20 milliamps
at a maximum operating voltage of 15 volts, we use Ohm's law, with a current of .020
amps and 15 volts minus the forward voltage drop one diode, 0.6 volts. This gives R1 =
(15-0.6)/.020 = 720 ohms. The nearest standard 5% resistor value is 750 ohms; 680 is a
bit risky at 15 volts. The power is (15-0.6)×.020 = 0.288 watts. These resistors will
rarely see this much power except when the case is opened in bright light.
R3 – 680 ohm 1/8 watt resistor. This limits the current to the handshaking pins, in the
event of a short circuit. It is irrelevant if these signals are not used or are never shorted.
C1 and C3 – 100 microfarad 20 volt (or more) electrolytic capacitors. The worst case for
the power supply is when 60 Hz AC power is used. We need to source 20 milliamps for
1/60th of a second (the interval between cycles on the power line). This is .02/60 = .00033
coulombs. 100 microfarads at 15 volts stores .0001×15 = .0015 coulombs. This gives us
a factor of 5 safety margin, so the ripple under full load should be under 3 volts. Note
that this full load condition will be rare.
C2, C4 – .01 microfarad 20 volt (or more) ceramic capacitors. The exact capacity is not
critical. The primary purpose of these capacitors is to eliminate radio frequency
interference from the RS-232 Data Diode power supply, and to filter out any radio
frequency signals that might be injected into the Data Diode from its power supply
connections. The larger capacitors C1 and C3 are electrolytic, and these typically do not
do a good job of filtering out radio-frequency signals.
D1 through D6 – silicon diodes. 1N1001 diodes work well. These must be able to
handle the inrush current during the charging of C1 and C3.
9
Intellectual Property Rights – Who owns the RS-232 Data Diode?
The RS-232 Data Diode was developed at the University of Iowa with funds from the
National Science Foundation under Grant No. CNS-052431 (ACCURATE) during 2006.
U.S. Patent 5,703,562, Method for transferring data from an unsecured computer to a
secured computer, granted Dec. 30, 1997 to Curt A. Nilsen, covers certain uses of the RS-
232 Data Diode. Specifically, this patent applies to the use of this (and other) data diodes
when the data transfer is from an insecure environment into a secure environment, which
is to say, the data diode is used to block the export of data from the secure environment.
This patent explicitly avoids claiming any rights over the use of data diodes for exporting
data from a secure environment to an insecure environment, for example, where the data
diode serves to prevent attacks against the secure system from the insecure system.
Data diodes and other devices for ensuring one-way flow of information have been
around for a long time. U.S. Patent 5,206,368, Signal isolating technique, granted June 3,
1980 to Bruce N. Lenderking, covers use of optical isolators in high fanout signal
distribution systems in order to prevent failures of one or another destination device from
preventing delivery of the signal to other destination devices. In effect, this patent is the
mirror of 5,703,562, since the signal source and all still-functional recipients are being
secured against failures in one or more recipients. If malicious behavior is accepted as a
type of failure, the expiration of this patent has expired may put most data diode
applications in the public domain.
The Owl Computing Technologies data diode uses fiber optics and two specially
configured ATM network interface cards. Inspection of this device to confirm that it is
incapable of sending data in the reverse direction is not at all trivial. Most of the Owl
data-diode documentation explicitly assumes the context of U.S. Patent 5,703,562,
speaking in terms of using the data diode to assure that nothing escapes from the secure
machine onto the Internet (http://www.owlcti.com/products.htm). It is
noteworthy, however, that the final suggested application in the Owl application notes
(http://www.owlcti.com/applications.htm) suggests using a data diode to
deliver content to an insecure web server from a secure content provider. At the very
least, this mention precludes anyone else patenting the use of Data Diodes for such a
purpose.
The Tenex Datagate Interactive Link Data Diode (IL-DD) is also documented in terms of
U.S. Patent 5,703,562 (http://www.tenix.com/Main.asp?ID=734). The
documentation speaks in terms of using the data diode to send data from a network of
uncertain security into a secure network Again, this device, although certified to a very
high level of security, is an opaque device, forcing the user to trust the certifying agency.
10
