Advantech ARK-3400, User Manual, Page: 76 / 87

ARK-3400 User Manual 64
C.1 RS-485 Auto Flow Control
The COM1 & COM2 port connector located on the rear face plate of ARK-3400 unit
can be configured to operate in RS-232, RS-422 or RS-485 mode by jumpers.
Figure C.1 BIOS COM port setting
C.1.1 Flow control, introduction
Consider the situation where someone is helping you harvest apples from a tree.
Your helper climbs up the tree and throws all the apples down to you. You have to put
them in buckets. In the normal situation, you can easily catch all the apples, but when
one bucket is full and it has to be replaced by an empty one, this action takes more
time than is available between two apples thrown by your helper.
Two different things can occur. Your helper stops until the new bucket is in position,
or some apples are damaged because they fall on the (rock hard, as it happens)
ground in the small period you are not able to catch them.
You would probably prefer the first method where your helper stops for a small
period. To achieve this, there will be some communication, eye-contact, a yell, or
something like that to stop him/her from throwing new apples. How simple, but is it
always this simple? Consider the situation where one computer device sends infor-
mation to another using a serial connection. Now and then, the receiver needs to do
some other actions, to write the contents of its buffers to disk for example. In this
period of time no new information can be received. Some communication back to the
sender is needed to stop the flow of bytes on the line. A method must be present to
tell the sender to pause. To do this, both software and hardware protocols have been
defined.
C.1.2 Software flow control
Both software and hardware flow control need software to perform the handshaking
task. This makes the term software flow control somewhat misleading. What is meant
is that with hardware flow control, additional lines are present in the communication
cable which signal handshaking conditions. With software flow control, which is also
known under the name XON-XOFF flow control, bytes are sent to the sender using
the standard communication lines.
Using hardware flow control implies, that more lines must be present between the
sender and the receiver, leading to a thicker and more expensive cable. Therefore,
software flow control is a good alternative if it is not needed to gain maximum perfor-
mance in communications. Software flow control makes use of the data channel