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6. DATA COMMUNICATIONS USING AN M
: N MULTIDROP LINK
MELSEC-A
6.
DATA COMMUNICATIONS USING AN M : N MULTIDROP LINK
An AJ71 C24 can perform data communications with several external devices
by constructing a multidrop link consisting of several external devices (m
stations) and several AJ71C24s (n stations). (The maximum number of m
and n stations is 32.)
This section describes how to do data communications using an m : n
multidrop link. This section only applies to m
:
n multilink data communica-
tions.
A computer link can be made with the full-duplex communications method
using the RS-232C and RS-422 lines by constructing an m
:
n multidrop link.
In addition, data transmission is initiated by a command from the external
device in the dedicated protocol.
(
The key points for constructing an m
:
n multidrop link system involve:
Methods of data communications
Conditions and methods for interlocking external devices (computers)
6.1
Key Points
(I) When using an m : n multidrop link system, only one computer can
perform data communications with a single PC CPU.
Set up the computers so that a computer and a PC CPU can do 1 : 1
communications. Sections 6.2 and 6.3 give the conditions and proce-
dures for computer interlocking.
1
(2) Data communications between a computer and a PC CPU can only be
done in the following way:
Full-duplex communications must be used. (m : n data communica-
tions cannot be done with half-duplex communications.)
Transmit a command from a computer using the dedicated protocol
(except for protocol 3). Data communications with protocol 3 and
data transmission from the sequence program using the on-demand
function cannot be done.
(3) All computers (including the computer that transmitted the data)
receive data from either computer. In addition, all computers receive
data transmitted from a PC CPU. Therefore, every computer that
receives data addressed to other stations (as specified by the station
number in the message) must ignore that data.
The AJ71C24 which is connected to the PC CPU ignores the receive
data which is addressed to other stations.
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