Sensoray 2426 - Manual (Page 14)

2426 Instruction Manual 12 Serial Communication Interface
Chapter 9: Serial Communication Interface
9.1 Overview
The 2426 module includes an asynchronous serial
communication port (comport) that can be configured to
operate with an RS-422 or RS-485 physical layer over
twisted-pair cable. Standard data rates are supported up to
115.2 Kbaud.
The comport can be accessed by a network client through the
module’s telnet server. After opening a telnet session to the
2426 module, the client attaches the session to the comport by
issuing a “COM” shell command. The server will respond by
sending a shell prompt and, from that point on, all incoming
comport traffic will be forwarded to the telnet client and all
characters sent to the telnet server will be transmitted on the
comport’s serial output.
COM lights briefly when a character is sent or received on
the comport interface.
At any given time the comport can be attached to only one
telnet session. When the comport is already attached to a telnet
session, attempts to attach it to another session (i.e., issuing a
“COM” command to a different shell) will fail and return an
error message.
It is not possible to access the comport through the module’s
embedded HTTP server.
Remote serial devices sometimes require binary data streams.
To accommodate such devices, the 2426 implements the Telnet
Binary Transmission Option. Refer to RFC 856 for details.
9.2 Configuration
9.2.1 Hardware Configuration
The comport’s physical layer is selected, and on-board line
termination resistors are enabled or disabled, by installing
hardware shunts onto the COM Configuration Posts (see
Figure 2). Shunts may be installed in any of the following
combinations (“ ” indicates installed shunt):
9.2.2 Software Configuration
Except for physical layer and line terminations, all comport
configuration settings are controlled through the comport’s
attached telnet session.
Some software configurable attributes (e.g., baud rate and
parity) are specified as arguments in the COM shell command.
These serve to establish an initial configuration for the comport
before it starts operating. In many applications, no additional
software configuration is required.
After issuing the COM shell command, all software
configurable attributes, as well as special control functions
such as BREAK transmission, can be dynamically controlled
by means of the Telnet Com Port Control (CPC) option. This
protocol is also used to receive notification of errors (e.g.,
parity, framing, overrun) and incoming BREAK signals, etc.
Refer to RFC 2217 for details.
Flow control between the comport and its network client is
managed through the Telnet CPC protocol. Flow control
between comport and remote serial devices is not supported.
9.3 COM Connector
A female DB-9 connector (COM connector in Figure 2) is used
to connect the comport to an external RS-422/RS-485 device
or network.
9.4 Wiring
Shielded cable is recommended for communication cables,
although unshielded cables may provide satisfactory
performance. If shielded cable is used, the shield conductor
should be connected to ground at one end of the cable and left
disconnected at the other end. This will provide the benefits of
shielding without introducing ground loops.
The 2426 comport can connect to a single remote serial device
or a network of remote devices. Some common device
network topologies and examples are discussed in the
following paragraphs. All of these examples assume the 2426
module is acting as a master on the device network.
Table 7: Valid Configuration Shunt Combinations
Shunts
Function
T
T
E
R
M
R
S
4
2
2
R
S
4
8
5
R
T
E
R
M
RS-485, unterminated
RS-485 with termination
RS-422, unterminated
RS-422 with TX termination only
RS-422 with RX termination only
RS-422 with TX and RX termination
Table 8: COM Port Connector Pinouts
Pin Name Signal Function
2 TX- Transmit, active low
3 RX- Receive, active low
5 GND Signal reference.
7 RX+ Receive, active high
8 TX+ Transmit, active high
1,4,6,9 NC
Not connected. These uncommitted pins are
reserved for future use.