TE Technology TC-36-25-RS485 - Manual (Page 43)

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APPENDIX C: Serial Communications
I. Definitions:
Note: where needed, the base of a number is indicated by its subscript. So, 20
10
is the number 20 in decimal (base 10)
format, and it converts to 14
16
in hexadecimal (base 16) format.
Use lowercase ASCII characters.
(stx)
Start of text character (*) or 2a
16
.
(etx) End of text character (carriage return) or 0d
16
.
(ack) Acknowledge character (^) or 5e
16
.
AA
Address characters are ASCII characters ranging from 0 through 9 or a through f.
CC
Command characters are ASCII characters from 0 through 9 or a through f.
DDDDDDDD
This is the ASCII representation of the hexadecimal return or send value. Negative numbers are
represented by the hex 2’s complement of the number (32‐bit word size):
1
10
= 00000001
16
‐1
10
=ffffffff
16
SS
The 8‐bit (modulo 256) hexadecimal checksum of characters sent to/from the controlling computer.
This is represented as 2 ASCII characters. The checksum calculation excludes the characters (stx), SS,
and (etx). The TC‐36‐25 RS485 sums (in an 8‐bit register) the hexadecimal values of the ASCII characters
sent to/from the controlling computer. Any overflow is truncated, leaving the 8‐bit (modulo 256)
checksum. When the controller is receiving data, this controller calculates the checksum and compares it
to the checksum sent by the controlling computer. The TC‐36‐25 RS485 will respond as shown below
depending on whether or not the checksums match. When the controlling computer is receiving data the
checksum sent by the controller can be used to make sure the data has not been received with an error.
Remember, when finding the ASCII value for a hexadecimal number (a, b, c, d, e, and f) be sure to
reference lower case letters.
To write a command to the controller, the controlling computer must send the following ASCII characters:
(stx)AACCDDDDDDDDSS(etx)
If the temperature controller receives the command and the checksum is correct, the temperature controller will respond
by sending back the “send value” and the checksum of those eight ASCII characters:
(stx)DDDDDDDDSS(ack)
To query a controller, there is no send value so the controlling computer only needs to send the following ASCII characters
with the “D”s filled with zeros:
(stx)AACC00000000SS(etx)
If the temperature controller receives the query and the checksum is correct, the temperature controller will respond by
sending back a “return value” and the checksum of those eight ASCII characters:
(stx)DDDDDDDDSS(ack)
If the checksum for a command or query is not correct the temperature controller will respond with eight upper case X’s
and then c0, which is the checksum of these eight X’s:
(stx)XXXXXXXXc0(ack)