Rabbit EM1500, Product Manual, Page: 107 / 120

EM1500 User’s Manual
www.rabbit.com
103
Flow Control
Flow control is a useful way of ensuring that connected devices are not overrun with more data than
they can handle. Software flow control may be employed with a 3-wire serial port. Hardware flow
control requires extra wires. Both ways require the 2 sides of the connection be in agreement about
which method to use. Whether implemented in software or hardware, flow control means that signals
are passed between 2 ends of the serial connection to say when to start and stop data transmission.
The EM1500 is very flexible in that flow control discipline is independently selectable for both data
flow directions.
MAC address
Media Access Control addresses are 48-bit numbers often written as a sequence of six two-digit hexa-
decimal numbers, separated by colons or hyphens; e.g., 00:90:C2:01:23:45. They are assigned to every
network device by the manufacturer and uniquely identify the Ethernet interface of the device.
The first 3 bytes of a Rabbit product are always:
00:90:C2
. The last 3 bytes are printed on a label
placed on the back of the EM1500 box. The 3 bytes are identified on the label as the serial number
(S/N), e.g., S/N 01.23.45.
MD5
This is an one-way hash function used to ensure message integrity (amongst other things).
Modem Server
A modem server allows multiple user connections at one time. The name originates from the fact that
it applies to multiple users of a modem, but it applies to other uses as well. E.g., a device that allows
multiple PCs access to a common printer.
Null-modem Cable
Connects the serial ports of 2 DTEs, e.g., 2 computers. Each DTE thinks it is talking to a DCE. This is
a typical configuration; many variations exist.
One-shot
A monostable multivibrator (aka, one-shot) when triggered, will produce an output pulse width that is
independent of the input pulse width. When used with a reset button, this device ensures that the sys-
tem will be held in a reset state for only so long, even if the reset button is held down indefinitely.
The one-shot device is used in the design of the EM1500 so that the reset line can be polled at certain
intervals to determine the operating mode of the unit.
RFC2217
This is an extension of the Telnet protocol to satisfy the needs of a class of functions that require the
capability of an asynchronous modem connection, e.g., a dial up connection to the Internet or a con-
nection to a bulletin board. This is known as outbound modem dialing, and is the primary purpose of
RFC2217.
A full understanding of this protocol requires reading it, probably multiple times. Basically, it allows
information regarding COM port configuration and modem line or signal changes to be communicated
between client and access server, as well as some flow control management.