SeaLevel ISO-COMM User manual

Sealevel Systems Inc. Telephone: [phone]
PO Box 830 Fax: [phone]
Liberty, SC 29657 USA www.sealevel.com
ISO-COMM
USER MANUAL
TM

Contents
INTRODUCTION ........................................................................ 1
OVERVIEW......................................................................................1
WHAT’S INCLUDED.........................................................................1
FACTORY DEFAULT SETTINGS ........................................................1
CARD SETUP ............................................................................ 2
ADDRESS SELECTION......................................................................2
PORT ENABLE / DISABLE ................................................................3
IRQ SELECTION..............................................................................3
INTERRUPT MODE OPTIONS ............................................................4
HEADERS E8 AND E9......................................................................6
INTERFACE OPTIONS.......................................................................7
RS-232 ........................................................................................7
RS-422 ........................................................................................7
Combined RS-232 and RS-422...................................................7
INSTALLATION ......................................................................... 8
OPERATING SYSTEM INSTALLATION ...............................................8
For Windows Users.....................................................................8
Other Operating Systems ............................................................8
SYSTEM INSTALLATION ..................................................................8
TECHNICAL DESCRIPTION ...................................................... 9
CONNECTOR PIN ASSIGNMENTS....................................................10
RS-422 (DB 9 Male) .............................................................10
RS-232 (DB 9 Male) .............................................................10
SPECIFICATIONS .................................................................... 11
ENVIRONMENTAL SPECIFICATIONS ...............................................11
MANUFACTURING.........................................................................11
POWER CONSUMPTION .................................................................11
MEAN TIME BETWEEN FAILURES (MTBF) ...................................11
PHYSICAL DIMENSIONS.................................................................11
APPENDIX A - TROUBLESHOOTING ...................................... 12
APPENDIX B - HOW TO GET ASSISTANCE............................ 14
APPENDIX C - ELECTRICAL INTERFACE .............................. 15
RS-232 .........................................................................................15
RS-422 .........................................................................................15
RS-485 .........................................................................................16
APPENDIX D - ASYNCHRONOUS COMMUNICATIONS ............ 17

APPENDIX E - SILK-SCREEN ................................................. 19
APPENDIX F - SCHEMATIC .................................................... 20
APPENDIX G - COMPLIANCE NOTICES ................................. 23
FEDERAL COMMUNICATIONS COMMISSION STATEMENT...............23
EMC DIRECTIVE STATEMENT ......................................................23
WARRANTY............................................................................ 24
Figures
Figure 1 - Address Selection Table ...........................................................2
Figure 2 - DIP-Switch Illustration ............................................................2
Figure 3 - Headers E1, E2, E3, and E4 (Factory Default) ......................3
Figure 4 - Header E5, Normal IRQ Mode................................................4
Figure 5 - Header E5, Shared IRQ Mode ................................................4
Figure 6 - Header E5, Sharing IRQs with another adapter ...................5
Figure 7 - IRQ Headers E1 - E5 (Factory Default) .................................5
Figure 8 - Headers E8 and E9 RS-485 Mode Enable ..............................6
Figure 9 - Four Port RS-232/RS-422 Configuration Table ....................7
Figure 10 - Asynchronous Communications Bit Diagram....................17
© Sealevel Systems, Inc.
SL9149 Revision 7/2006
Sealevel Systems, Incorporated. All rights reserved.

Introduction
Sealevel Systems ISO-COMM Page 1
Introduction
Overview
The Sealevel Systems ISO-COMM provides the PC with additional ground
isolated serial ports for terminals, modems, printers, etc. Isolation is important in
installations where the equipment being connected to the PC is either far from
the PC, or on a different power transformer circuit. Ground loop current is a
commonly neglected and misunderstood phenomena that leads to the failure (and
destruction) of communication interfaces. The ISO-COMM isolated boards
provide up to 500 volts DC of isolation.
The ISO-COMM provides two or four DOS and Windows compatible serial
ports, and also offers extended ‘AT’ interrupts. Each port is independently
addressable as COM1: through COM4:, or any other I/O address up to 3FF Hex.
What’s Included
The ISO-COMM is shipped with the following items. If any of these items are
missing or damaged, contact the supplier.
• ISO-COMM Serial I/O Adapter
• Cable providing 2 DB-9 connectors.
• Sealevel Software
Factory Default Settings
The ISO-COMM factory default settings are as follows:
Port # Base Address IRQ
Port 1 3F8 4
Port 2 2F8 3
Port 3 3E8 4
Port 4 2E8 3
For your reference, record installed ISO-COMM settings below:
Port # Base Address IRQ
Port 1
Port 2
Port 3
Port 4

Card Setup
Sealevel Systems ISO-COMM Page 2
Card Setup
The ISO-COMM contains several jumper straps which must be set for proper
operation.
Address Selection
Each port on the ISO-COMM occupies 8 consecutive I/O locations. A
DIP-switch is used to set the base address for these locations. Be careful when
selecting the base address as some selections conflict with existing ports. The
following table shows several examples that typically do not cause a conflict.
SW4 sets the I/O address for port 1, SW3 sets the address for port 2, SW2 sets
the address for port 3 and SW1 sets the address for port 4.
Address Binary Switch Position Setting
Hex A9 A0 1 2 3 4 5 6 7
280-287 1010000XXX Off On Off On On On On
2A0-2A7 1010100XXX Off On Off On Off On On
2E8-2EF 1011101XXX Off On Off Off Off On Off
2F8-2FF 1011111XXX Off On Off Off Off Off Off
3E8-3EF 1111101XXX Off Off Off Off Off On Off
300-307 1100000XXX Off Off On On On On On
328-32F 1100101XXX Off Off On On Off On Off
3F8-3FF 1111111XXX Off Off Off Off Off Off Off
Figure 1 - Address Selection Table
The following illustration shows the correlation between the DIP-switch setting
and the address bits used to determine the base address. In the example below,
address 300 is selected as a base. Address 300 in binary is XX11 0000 0XXX
where X = a non-selectable address bit.
1 2 3 4 5 6 7
ON
OFF
A9
A
3
8
E
Figure 2 - DIP-Switch Illustration
Note: Setting the switch ‘On’ or ‘Closed’ corresponds to a ‘0’ in the address,
while leaving it ‘Off’ or ‘Open’ corresponds to a ‘1’.
Refer to Appendix A for common address contentions.

Card Setup
Sealevel Systems ISO-COMM Page 3
Port Enable / Disable
Each port on the ISO-COMM can be enabled or disabled with switch position 8
on the DIP-switch. The port is enabled with the switch ‘On’ or ‘Closed’ and
disabled when ‘Off’ or ‘Open’. If any port is disabled, be sure to disable the
interrupt request for that port by removing the IRQ jumper.
IRQ Selection
Headers E1, E2, E3 and E4 select the IRQ for each serial port. If COM1: is
selected, this jumper must be on the IRQ4 setting. If COM2: is selected, this
jumper must be on IRQ3.
2
3
4
5
7
10
11
12
15
2
3
4
5
7
10
11
12
15
2
3
4
5
7
10
11
12
15
E3 - Port2
E4 - Port1
E2 - Port3
E1 - Port4
2
3
4
5
7
10
11
12
15
Figure 3 - Headers E1, E2, E3, and E4 (Factory Default)
Note: Most communications software applications default COM3: to IRQ4 and
COM4: to IRQ3. This requires the sharing of interrupts between COM1:
and COM3:, and between COM2: and COM4:. While this is the default, it
is not always the preferred setting. Check your software configuration
instructions to determine the most appropriate IRQ selection.

Card Setup
Sealevel Systems ISO-COMM Page 4
Interrupt Mode Options
Header E5 selects the interrupt mode for each port. Each port must be set in the
correct mode to insure proper installation.
‘N’ indicates the (N)ormal, single interrupt per port mode. ‘S’ Indicates the
(S)hared interrupt mode, which allows more than one port to access a single
IRQ. Any two or more ports can share a common IRQ by placing the jumpers on
the same IRQ setting, and setting the appropriate selections at E5. Consult your
particular software for IRQ selection. If no interrupt is desired, remove the
jumper. ‘M’ indicates the inclusion of a 1K ohm pull-down resistor required on
one port when sharing interrupts.
M
S
N
M
S
N
E5
12
3
4
Figure 4 - Header E5, Normal IRQ Mode
Set jumpers to ‘S’ for shared interrupt mode on all blocks sharing an IRQ except
one. Set that port block for ‘M’. This provides the pull-down resistor circuit that
makes sharing of IRQs possible. If you are using more than one ISO-COMM or
a compatible card in a bus you should only have one port set to ‘M’. The
following example shows all four ports sharing a single IRQ.
M
S
N
M
S
N
E5
12
34
Figure 5 - Header E5, Shared IRQ Mode

Card Setup
Sealevel Systems ISO-COMM Page 5
Set jumper to ‘S’ if you are using more than one ISO-COMM in a bus or you
wish to completely remove the pull-down resistor for hardware compatibility.
Setting the board in this configuration when it is not accompanied by a pull-
down resistor will prevent the ports from triggering an interrupt.
M
S
N
M
S
N
E5
12
34
Figure 6 - Header E5, Sharing IRQs with another adapter
2
3
4
5
7
10
11
12
15
2
3
4
5
7
10
11
12
15
2
3
4
5
7
10
11
12
15
E3 - Port2
E4 - Port1
E2 - Port3
E1 - Port4
2
3
4
5
7
10
11
12
15
M
S
N
M
S
N
E5
12
34
Figure 7 - IRQ Headers E1 - E5 (Factory Default)
The above example shows the correlation between jumper block E5 and the
interrupt jumpers (E1-E4). This example shows the ISO-COMM shipping
configuration with COM1: & COM3: sharing interrupt 4 and COM2: & COM4:
sharing interrupt 3.

Card Setup
Sealevel Systems ISO-COMM Page 6
Headers E8 and E9
RS-485 is backwardly compatible with RS-422, however it is optimized for
partyline or multi-drop applications. The output of the RS-422/485 driver is
capable of being active (enabled) or tri-state (disabled). This capability allows
multiple PCs to be connected in a multi-drop bus and selectively polled. Half-
duplex two-wire operation is also possible by connecting TX+ to RX+ and TX-
to RX- in your cable hood. The enable to the driver is connected to the UART
Request To Send (RTS) line for RS-485 communications. This allows the RS-
485 driver to be tri-stated when inactive on a multi-drop polled network. The
software that you are using must "know how" to enable the driver when it is
answering a poll. Failure to correctly set this jumper can cause transmitter
contention problems preventing operation by any nodes on the network. Sealevel
Systems communication software is compatible with this mode of operation. To
enable the driver with RTS install jumpers at E9-A for port 1, E9-C for port 2,
E8-A for port 3 and E8-C for port 4.
The RS-485 ‘Echo’ is the result of connecting the receiver inputs to the
transmitter outputs. Every time a character is transmitted; it is also received. This
can be beneficial if the software can handle the ‘echoing’ (i.e. using received
characters to throttle the transmitter) or it can confuse the system if the software
does not. The RS-485 echo can also be disabled at E8 and E9. To disable the
echo install a jumper at E9-B for port 1, E9-D for port 2, E8-B for port 3 and
E8-D for port 4.
The following example shows the driver in the RS-485 mode and the echo
enabled.
E8 & E9 D
C
B
A
Figure 8 - Headers E8 and E9 RS-485 Mode Enable

Card Setup
Sealevel Systems ISO-COMM Page 7
Interface Options
Due to the versatility of the ISO-COMM series a wide range of interface
configuration options are available to the end user. Please use this section as a
guide in configuring your board to provide the interface you require.
RS-232
To configure the board as RS-232 insure that the 75174 driver at U33 & U27
and the DIP resistor pack at RP15 have been removed (if previously installed).
By installing the MAX234 driver at U34 & U28, the board will be configured as
RS-232. Please note that on two port versions of the ISO-COMM series only
one driver chip will be provided at either U33 or U34.
RS-422
To configure the ISO-COMM as RS-422 insure that the MAX234 driver at U34
& U28 have been removed (if previously installed). By installing the 75174
driver at U33 & U27 and installing a 100 ohm DIP resistor at RP-15, the board
will be configured as RS-422. Please note that on two port versions of the
ISO-COMM series only one driver chip will be provided at either U33 or U34.
Combined RS-232 and RS-422
On the four port ISO-COMM boards two ports of RS-232 and two ports of RS-
422 is possible, but it is complicated to configure. The configuration requires
that you selectively bend (or float) pins on the DIP-resistor pack out of the
socket.
Configuration Drivers RP-15 Pin Positions
Ports 1 & 2 RS-232
Ports 3 & 4 RS-422
U34 & U27 Installed
U33 & U28 Removed
Pins 1, 7, 8, 4 "Floated"
All Others Installed
Ports 1 & 2 RS-422
Ports 3 & 4 RS-232
U33 & U28 Installed
U34 & U27 Removed
Pins 2, 3, 5, 6 "Floated"
All Others Installed
Figure 9 - Four Port RS-232/RS-422 Configuration Table

Installation
Sealevel Systems ISO-COMM Page 8
Installation
Operating System Installation
For Windows Users
Start by choosing Install Software at the beginning of the CD. Choose
Asynchronous COM: Port Software, SeaCOM.
Other Operating Systems
Refer to the appropriate section of the Serial Utilities Software.
System Installation
The ISO-COMM can be installed in any of the PC expansion slots, but to
access the ‘AT’ or (E)ISA IRQs (10, 11, 12, 15) the adapter must be installed in
one of the 16 bit slots. The ISO-COMM contains several jumper straps for each
port which must be set for proper operation prior to installing the card into the
computer.
1. Turn off PC power. Disconnect the power cord.
2. Remove the PC case cover.
3. Locate an available slot and remove the blank metal slot cover.
4. Gently insert the ISO-COMM into the slot. Make sure that the adapter is
seated properly.
5. Replace the screw.
6. Install the twin DB-9 (ports 3 and 4) Cable
7. Replace the cover.
8. Connect the power cord.
Installation is complete.

Technical Description
Sealevel Systems ISO-COMM Page 9
Technical Description
The Sealevel Systems ISO-COMM provides the PC with two or four additional
ground isolated RS-422/485/232 or two or four non-isolated RS-422/485 serial
ports for terminals, modems, printers, etc. Isolation is important in installations
where the equipment being connected to the PC is either far from the PC, or on a
different power transformer circuit. Ground loop current is a commonly
neglected and misunderstood phenomena that leads to data loss and the
destruction of communications interfaces. The ISO-COMM isolated boards
provide up to 500 volts DC of isolation.
The ISO-COMM utilizes the 16550 UART. This chip features programmable
baud rate, data format, interrupt control and a 16 byte input and output FIFO.
Also available as an option is the 16C650 UART that provides a deeper FIFO
(32 bytes) and enhanced clocking features.
• Two / four serial ports with RTS and CTS modem control signals
• Individually selectable port addresses
• Individually selectable interrupt (2-7, 10, 11, 12, and 15 )
• Multiple cards can share the same IRQ signal
• DB-9 male connectors
• Software available for interrupt buffering
• ISO-COMM-1 Four Port Isolated RS-422/485
Part # 3415
Part # 3415-232 (includes RS-232 chips)
• ISO-COMM-2 Four Port Non-Isolated RS-422/485
Part # 3416 (options same as 3415)
• ISO-COMM-3 Two Port Isolated RS-422/485
Part # 3417
Part # 3417-232 (includes RS-232 Chip)
• ISO-COMM-4 Four Port Isolated RS-232
Part # 3418
• ISO-COMM-5 Two Port Isolated RS-232
Part # 3419
Note: Part # 34xx-232 specifies RS-232 chips to be included for mixed RS-232
and RS-422 on same board.

Technical Description
Sealevel Systems ISO-COMM Page 10
Connector Pin Assignments
RS-422 (DB 9 Male)
Signal Name Pin # Mode
GND Ground 5
TX + Transmit Data Positive 4 Output
TX- Transmit Data Negative 3 Output
RTS+ Request To Send Positive 6 Output
RTS- Request To Send Negative 7 Output
RX+ Receive Data Positive 1 Input
RX- Receive Data Negative 2 Input
CTS+ Clear To Send Positive 9 Input
CTS- Clear To Send Negative 8 Input
RS-232 (DB 9 Male)
Signal Name Pin # Mode
GND Ground 5
TD Transmit Data 3 Output
RTS Request To Send 7 Output
RD Receive Data 2 Input
CTS Clear To Send 8 Input
Note: These are the only pins that should be connected in your cable.
Connecting any other pins on the DB-9 connector could result in
intermittent data.
Technical Note: Please terminate any control signals that are not going to be
used. The most common way to do this is connect RTS to CTS and RI. Also,
connect DCD to DTR and DSR. Terminating these pins, if not used, will help
insure you get the best performance from your adapter.

Specifications
Sealevel Systems ISO-COMM Page 11
Specifications
Environmental Specifications
Specification Operating Storage
Temperature
Range
0º to 50º C
(32º to 122º F )
-20º to 70º C
(-4º to 158º F)
Humidity Range 10 to 90% R.H.
Non-Condensing
10 to 90% R.H.
Non-Condensing
Manufacturing
• All Sealevel Systems Printed Circuit boards are built to U.L. 94V0 rating
and are 100% electrically tested. These printed circuit boards are solder
mask over bare copper or solder mask over tin nickel.
Power Consumption
Part # +5 V Dc Supply line
3415 880mA
3416 450mA
3417 450mA
3418 600mA
3419 450mA
Mean Time Between Failures (MTBF)
Greater than 150,000 hours. (Calculated)
Physical Dimensions
Board length 13.33 inches (33.86 cm)
Board Height including Goldfingers 4.2 inches (10.66 cm)
Board Height excluding Goldfingers 3.9 inches (9.91 cm)

Appendix A - Troubleshooting
Sealevel Systems ISO-COMM Page 12
Appendix A - Troubleshooting
Serial Utility test software is supplied with the Sealevel Systems adapter and will
be used in the troubleshooting procedures. By using this software and following
these simple steps, most common problems can be eliminated without the need
to call Technical Support.
1. Identify all I/O adapters currently installed in your system. This includes
your on-board serial ports, controller cards, sound cards etc. The I/O
addresses used by these adapters, as well as the IRQ (if any) should be
identified.
2. Configure your Sealevel Systems adapter so that there is no conflict with
currently installed adapters. No two adapters can occupy the same I/O
address.
3. Make sure the Sealevel Systems adapter is using a unique IRQ The IRQ is
typically selected via an on-board header block. Refer to the section on Card
Setup for help in choosing an I/O address and IRQ.
4. Make sure the Sealevel Systems adapter is securely installed in a
motherboard slot.
5. When running DOS, Windows 3.x or other operating systems refer to the
Serial Utilities software for that operating system and the User Manual to
verify that the Sealevel Systems adapter is configured correctly. The
supplied software contains a diagnostic program 'SSD' that runs under DOS
and will verify if an adapter is configured properly. This diagnostic program
is written with the user in mind and is easy to use. Refer to the DIAG.txt file
in the dos\diag directory for detailed instructions on using 'SSD'.
6. For Windows 95/98 and Windows NT, the diagnostic tool 'WinSSD' is
installed in the Sealevel folder on the Start Menu during the setup process.
First find the ports using the Device Manager, then use 'WinSSD' to verify
that the ports are functional.
7. Always use the Sealevel Systems diagnostic software when troubleshooting
a problem. This will help eliminate any software issues and identify any
hardware conflicts.

Appendix A - Troubleshooting
Sealevel Systems ISO-COMM Page 13
8. The following are known I/O conflicts:
• The 278 and 378 settings may conflict with your printer I/O adapter.
• 3B0 cannot be used if a Monochrome adapter is installed.
• 3F8-3FF is typically reserved for COM1:
• 2F8-2FF is typically reserved for COM2:
• 3E8-3EF is typically reserved for COM3:
• 2E8-2EF is typically reserved for COM4:
9. Remember to use Sealevel Systems diagnostic software when
troubleshooting a problem. This will help eliminate any software issues and
identify any hardware conflicts.

Appendix B - How To Get Assistance
Sealevel Systems ISO-COMM Page 14
Appendix B - How To Get Assistance
Please refer to Troubleshooting Guide prior to calling Technical Support.
1. Begin by reading through the Trouble Shooting Guide in Appendix
A. If assistance is still needed please see below.
2. When calling for technical assistance, please have your user
manual and current adapter settings. If possible, please have the
adapter installed in a computer ready to run diagnostics.
3. Sealevel Systems provides an FAQ section on its web site. Please
refer to this to answer many common questions. This section can be
found at http://www.sealevel.com/faq.htm .
4. Sealevel Systems maintains a Home page on the Internet. Our
home page address is www.sealevel.com. The latest software
updates, and newest manuals are available via our FTP site that can
be accessed from our home page.
5. Technical support is available Monday to Friday from 8:00 a.m. to
5:00 p.m. eastern time. Technical support can be reached at (864)
843-4343.
RETURN AUTHORIZATION MUST BE OBTAINED FROM SEALEVEL
SYSTEMS BEFORE RETURNED MERCHANDISE WILL BE
ACCEPTED. AUTHORIZATION CAN BE OBTAINED BY CALLING
SEALEVEL SYSTEMS AND REQUESTING A RETURN
MERCHANDISE AUTHORIZATION (RMA) NUMBER.

Appendix C - Electrical Interface
Sealevel Systems ISO-COMM Page 15
Appendix C - Electrical Interface
RS-232
Quite possibly the most widely used communication standard is RS-232. This
implementation has been defined and revised several times and is often referred
to as RS-232 or EIA/TIA-232. The IBM PC computer defined the RS-232 port
on a 9 pin D sub connector and subsequently the EIA/TIA approved this
implementation as the EIA/TIA-574 standard. This standard is defined as the
9-Position Non-Synchronous Interface between Data Terminal Equipment and
Data Circuit-Terminating Equipment Employing Serial Binary Data
Interchange. Both implementations are in wide spread use and will be referred to
as RS-232 in this document. RS-232 is capable of operating at data rates up to
20 Kbps at distances less than 50 ft. The absolute maximum data rate may vary
due to line conditions and cable lengths. RS-232 often operates at 38.4 Kbps
over very short distances. The voltage levels defined by RS-232 range from -12
to +12 volts. RS-232 is a single ended or unbalanced interface, meaning that a
single electrical signal is compared to a common signal (ground) to determine
binary logic states. A voltage of +12 volts (usually +3 to +10 volts) represents a
binary 0 (space) and -12 volts (-3 to -10 volts) denotes a binary 1 (mark). The
RS-232 and the EIA/TIA-574 specification defines two type of interface circuits,
Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment
(DCE). The Sealevel Systems adapter is a DTE interface.
RS-422
The RS-422 specification defines the electrical characteristics of balanced
voltage digital interface circuits. RS-422 is a differential interface that defines
voltage levels and driver/receiver electrical specifications. On a differential
interface, logic levels are defined by the difference in voltage between a pair of
outputs or inputs. In contrast, a single ended interface, for example RS-232,
defines the logic levels as the difference in voltage between a single signal and a
common ground connection. Differential interfaces are typically more immune to
noise or voltage spikes that may occur on the communication lines. Differential
interfaces also have greater drive capabilities that allow for longer cable lengths.
RS-422 is rated up to 10 Megabits per second and can have cabling 4000 feet
long. RS-422 also defines driver and receiver electrical characteristics that will
allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels
range from 0 to +5 volts. RS-422 does not define a physical connector.

Appendix C - Electrical Interface
Sealevel Systems ISO-COMM Page 16
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for
partyline or multi-drop applications. The output of the RS-422/485 driver is
capable of being Active (enabled) or Tri-State (disabled). This capability allows
multiple ports to be connected in a multi-drop bus and selectively polled.
RS-485 allows cable lengths up to 4000 feet and data rates up to 10 Megabits
per second. The signal levels for RS-485 are the same as those defined by
RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32
receivers to be connected to one line. This interface is ideal for multi-drop or
network environments. RS-485 tri-state driver (not dual-state) will allow the
electrical presence of the driver to be removed from the line. Only one driver
may be active at a time and the other driver(s) must be tri-stated. The output
modem control signal RTS controls the state of the driver. Some communication
software packages refer to RS-485 as RTS enable or RTS block mode transfer.
RS-485 can be cabled in two ways, two wire and four wire mode. Two wire
mode does not allow for full duplex communication, and requires that data be
transferred in only one direction at a time. For half-duplex operation, the two
transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx-
to Rx-). Four wire mode allows full duplex data transfers. RS-485 does not
define a connector pin-out or a set of modem control signals. RS-485 does not
define a physical connector.

Appendix D - Asynchronous Communications
Sealevel Systems ISO-COMM Page 17
Appendix D - Asynchronous Communications
Serial data communications implies that individual bits of a character are
transmitted consecutively to a receiver that assembles the bits back into a
character. Data rate, error checking, handshaking, and character framing
(start/stop bits) are pre-defined and must correspond at both the transmitting and
receiving ends.
Asynchronous communications is the standard means of serial data
communication for PC compatibles and PS/2 computers. The original PC was
equipped with a communication or COM: port that was designed around an 8250
Universal Asynchronous Receiver Transmitter (UART). This device allows
asynchronous serial data to be transferred through a simple and straightforward
programming interface. Character boundaries for asynchronous communications
are defined by a starting bit followed by a pre-defined number of data bits (5, 6,
7, or 8). The end of the character is defined by the transmission of a pre-defined
number of stop bits (usually 1, 1.5 or 2). An extra bit used for error detection is
often appended before the stop bits.
1
1.5
2
Parity
Bit
5 to 8 Data Bits
Odd
Even
or
Unused
Remain Idle
or
Next Start Bit
1
0
Idle State
of
Line
Figure 10 - Asynchronous Communications Bit Diagram
This special bit is called the parity bit. Parity is a simple method of determining
if a data bit has been lost or corrupted during transmission. There are several
methods for implementing a parity check to guard against data corruption.
Common methods are called (E)ven Parity or (O)dd Parity. Sometimes parity is
not used to detect errors on the data stream. This is referred to as (N)o parity.