Advantech ARK-3440 Series, Руководство пользователя

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ARK-3440 User Manual 52
One is, where the RTS of each side is connected with the CTS side of the other. In
that way, the communication protocol differs somewhat from the original one. The
RTS output of computer A signals computer B that A is capable of receiving informa-
tion, rather than a request for sending information as in the original configuration.
This type of communication can be performed with a null modem cable for full hand-
shaking. Although using this cable is not completely compatible with the original way
hardware flow control was designed, if software is properly designed for it, it can
achieve the highest possible speed because no overhead is present for requesting
on the RTS line and answering on the CTS line.
In the second situation of null modem communication with hardware flow control, the
software side looks quite similar to the original use of the handshaking lines. The
CTS and RTS lines of one device are connected directly to each other. This means,
that the request to send query answers itself. As soon as the RTS output is set, the
CTS input will detect a high logical value indicating that sending of information is
allowed. This implies, that information will always be sent as soon as sending is
requested by a device if no further checking is present. To prevent this from happen-
ing, two other pins on the connector are used, the data set ready DSR and the data
terminal ready DTR. These two lines indicate if the device attached is working prop-
erly and willing to accept data. When these lines are cross-connected (as in most null
modem cables) flow control can be performed using these lines. A DTR output is set,
if that computer accepts incomming characters.
C.1.4 How to Implement
Implementing proper flow control can give some headaches. The main problems are
the numerous ways it can be done and especially for null modem connections, the
lack of a standard way of doing. The best way to implement rigid flow control in your
software is to use preprogrammed routines from a reliable source. The problems
involved in the own development of communication routines is often not worth the
effort compared to the relative low prices of professional communication libraries. A
good library is the COMM-DRV/Lib from Willies Computer Software Co. This library
supports all versions of Windows and MS-DOS. XModem, YModem and ZModem file
transfer routines are provided and all source code is included. Includes also Modem
handling and string handling routines.
C.2 WOL Setting
C.2.1 Introduction
Wake on LAN (WOL, sometimes WoL) is an Ethernet computer networking standard
that allows a computer to be turned on or woken up remotely by a network message.