TE Technology TC-24-25 - Manual (Page 22)

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APPENDIX F Serial Communications
I. Definitions:
All numeric values are in hexadecimal format.
Use lowercase ASCII characters.
(stx)
Start of text character (*) 2a hex.
(etx) End of text character (carriage return) or 0d hex.
(ack) Acknowledge character (^) 5e hex.
AA
Address characters are ASCII characters from 0 through 9 or a through f.
00 is universal address of all controllers when using a chain of the RS-485 based
devices.
CC
Command characters are ASCII characters from 0 through 9 or a through f.
DDDDDDDD Hex two’s-complement return or send value.
1d = 00000001
-1d=ffffffff
SS
The 8-bit (modulo 256) checksum of characters sent to/from the controlling computer.
This is represented as 2 ASCII hex characters. The checksum calculation excludes the
characters (stx), SS, (etx). The TC-24-25 sums (in an 8-bit register) the ASCII values of
the 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 number (as
represented by two hex characters) is compared to the two ASCII character hex checksum
sent by the controlling computer. The TC-24-25 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 hex number (a, b,
c, d, e, and f) be sure to use lower case letters.
To write a command to a 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:
(stx)AACCSS(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)