Black Box 945 - Manual (Page 12)

How to Use Data Communications
12
WATLOW Series 945
ASCII
Example Format
For your benefit, we're presenting message/response examples with syntax re-
quired for Series 945 communication. Information bracketed by < > indicates a
description, rather than literal characters. We show each ASCII character that you
must transmit to the Series 945, including space between the characters. (A
"space" is itself an ASCII character, hex 20). For clarity, we also represent each
ASCII character as a hexadecimal pair. The pairs are spread apart on the page for
easy reading. However, electronic devices "see" the hex pairs all together in
"strings," with no spaces in between.
For instance, from the example just below, you want to set the Alarm 1 Low
(A1LO) parameter to 500
°
. Notice the syntax just below which uses the "=" com-
mand. = <Space> A1LO <Space> 500
Figure 5 -
Series 945
General Message
Syntax Example.
ASCII
Characters
HEX
Value
1
Space
5
0
0
41 4C 31 20 35 30 30
<Space>
=
3D 20
A
L
<CR>
0D 4F
O
To send this message, key the ASCII characters into your computer, or write them
into your program. The computer, in turn, will send a string similar to the one at the
bottom of the example, 3D2041314C4F20353030.
Notice that we haven't mentioned protocol here, or any characters added to this
syntax by a protocol. With XON/XOFF, the message above can be transmitted
with only an additional Carriage Return <CR> (hex 0D) character at the end.
However, the ANSI X3.28 Protocol requires an envelope of Start of Text <STX>
(hex 02) and End of Text <ETX> (hex 03) characters around the information you
see above. You'll learn how to do that in the pages ahead.
XON/XOFF Protocol for RS-423A
XON/XOFF (flow control) Protocol allows a communicating device (either a 945 or
the host) to suspend transmission of all messages from the other device, and then
to continue transmission when it's again ready.
The device that needs to suspend transmission sends the XOFF character
(hex 13) to stop the other device's transmitter, and XON (hex 11) to restart it. Note
that technically any character will restart the transmitter, but only the XON character
is not a part of any regular message that may be transferring.
Messages transmit according to the syntax described in the XON/XOFF formats
which follow for each command.
The XON/XOFF Protocol requires a Carriage Return character
(hex 0D) at the end of every message.