User`s manual

EM1500 User’s Manual www.rabbit.com 95
the appropriate network protocol. Such a device driver is usually called a “COM port redirector,” after the
DOS naming convention for serial ports, i.e., COM1, COM2 etc. When the redirector is installed, a large
number of virtual serial ports is made available, e.g., COM5, COM6 up to COM256 or more.
The last protocol that the EM1500 supports is intended for inter-EM1500 communication. It is an exten-
sion to the RFC2217 protocol to allow for the specific and useful features of the EM1500, especially pack-
etization. The protocol extension is transparently negotiated. If the EM1500 is in fact talking to an
ordinary host (not another EM1500) then the protocol will be downgraded to normal RFC2217 without
adversely affecting the host.
The modem server protocol requires greater network bandwidth, however the extra traffic is usually negli-
gible. One possible exception to keep in mind is that data bytes with a value of 255 (or hex FF) have to be
transmitted as two bytes of value 255. Thus, in the worst case of transmitting nothing but value 255 char-
acters, the required network bandwidth is doubled.
Note that you can freely mix modem server and Rabbit extensions; however, you cannot have one end of
the network configured for raw mode and the other for modem server mode. If so, the data stream will be
corrupted.
If you are using some of the more advanced features of the EM1500, like packetization and protocol trans-
lation, then you will have to use Rabbit extensions. Without this setting, the additional information (packet
boundaries etc.) will be lost. In practice, this means that packetization is only supported when you are
using an EM1500 at each end of the network connection. You can, however, write host code to support the
Rabbit extension of the RFC2217 protocol. Contact Rabbit technical support for details. The extension
specifications are openly available at no cost.
B.2.1 Packetization
Many serial protocols use the concept of discrete messages, as opposed to the more common data stream
approach. The EM1500 includes support for message-based protocols, associated with the term “packeti-
zation.” Other terms in common use are “records,” “segments” or “frames,” but we use “packets” in this
document.
The Protocol configuration tab allows packetization options to be specified. Since this is an extension to
the standard protocol (RFC2217), it is necessary to check the “Rabbit extensions” option. Checking this
option includes a minor modification to the standard protocol which allows messages to be delimited.
In practice, packetization is most useful when two EM1500s are being used as a serial bridge or extender.
Most PC software will not be able to understand the packetizing extensions; however, it is relatively easy
to write specialized PC software that takes advantage of this feature.