Watlow Series 942 Data Communications - Manual (Page 15)

How to Use Data Communications
15
WATLOW Series 942
ANSI X3.28
ANSI X3.28 Protocol for RS-422A and EIA-485
The ANSI X3.28 Protocol provides high quality communications by requiring a
response to every message. With a multiple device or "multidrop" network, this
protocol prevents confusion among the separate devices. Furthermore, if noise
occurs somewhere in the system, no parameter will change because noise
can't comply with the protocol.
By placing messages inside a protocol envelope, the messages are protected.
In the examples to come you'll see how this works.
The ANSI X3.28 Protocol requires STX characters at the beginning of a
message and ETX characters at the end.
34 05
4 ASCII
Characters
HEX Value
4
34 06
ASCII
Characters
HEX Value
Starting Communications in ANSI X3.28 Protocol
Here's the syntax for starting communications with ANSI X3.28 Protocol. The
master device, your computer, must initiate the data link. The example below
uses the ASCII number 4 as a Series 942 device address.
Enter in ASCII, using this syntax:
Response from the 942:
Address ASCII Equivalent
0 - 9 0 - 9
10 - 31 A - V
Device Address
If you are using the ANSI X3.28 Protocol, you must have a device address
(identification) number. A Watlow RS-422A multidrop network can handle up to
10 devices with this protocol. EIA-485 can handle up to 32 devices. Set the
address number with the Series 942 in the Addr parameter under the Setup
menu.
Table 3 -
Address to ASCII
Conversion.