User's Manual

MDS 05-2806A01, Rev. E.1 MDS iNET 900 Series User’s Guide 57
between serial and IP remote devices. Two common scenarios are PC
applications using IP to talk to remote devices, and serial PC applica-
tions talking to remote serial devices over an IP network.
Essentially the same data services are available for both serial ports:
COM1 and COM2. Note that the transceiver’s COM1 port is DCE and
COM2 is DTE. Therefore, if the RTU to be connected is also DTE, then
a null-modem cable will need to be used when connecting to
COM2.
NOTE: In the discussion that follows, COM1 and COM2 will be treated
alike unless noted.
Com1 Port–Dual Purpose Capability
The COM1 port is used as a local console connection point and to pass
serial data with an external device. Setting the
COM1 port status to Enable
prevents access to the Management System (MS) through this port.
However, the MS can still be accessed via the LAN port using Telnet or
a web browser.
To restore the COM1 port to support Management System services,
connect a terminal to the port, select the proper baud rate (19,200 is
default), and enter an escape sequence (
+++) to reset it to the console
mode.
TCP vs. UDP
Both types of IP services are used by the transceiver embedded serial
device server—TCP and UDP. TCP provides a connection-oriented link
with end-to-end acknowledgment of data, but with some added over-
head. UDP provides a connectionless best-effort delivery service with
no acknowledgment.
Most polled protocols will be best served by UDP service as the protocol
itself has built-in error recovery mechanisms. UDP provides the needed
multidrop operation by means of multicast addressing.
On the other hand, TCP services are best suited for applications that do
not have a recovery mechanism (error-correction) and must have the
guaranteed delivery that TCP provides despite the extra overhead. The
IP-to-Serial example shows how to do this. (See “IP-to-Serial Applica-
tion Example” on Page 67.)
Serial Encapsulation
Transparent encapsulation, or IP tunneling, provides a mechanism to
encapsulate serial data into an IP envelope. Basically, all the bytes
received through the serial port are put into the data portion of a TCP or
UDP packet (TCP or UDP are user configurable options). In the same
manner, all data bytes received in a TCP or UDP packet are output
through the serial port.