TE Technology TC-720 - Manual (Page 74)

Appendix B - USB Communications
I. Definitions:
All numeric values are in hexadecimal format. Use lowercase ASCII characters.
Note: Where applicable, 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.
(stx) Start of text character (* ) 2a
16
.
(etx) End of text character (carriage return) or 0d
16
.
(ack) Acknowledge character (^) 5e
16
.
CC Command characters are ASCII characters from 0 through 9 or a through f.
DDDD 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 (16-bit word size):
1
10
= 0001
16
-1
10
= ffff
16
SS The 8 bit (modulo 256) checksum of characters sent/from the controlling computer. This is represented as
two ASCII hex characters. The checksum excludes the characters (stx), SS, and (etx). The TC-720 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-720 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)CCDDDDSS(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 four ASCII data characters, DDDD:
(stx)DDDDSS (ack)
To query a controller, there is no send value so the controlling computer only needs to send the ASCII command characters
with the “D”s filled with zeros:
(stx)CC0000SS(etx)
If the checksum for a query is correct the temperature controller will respond by sending back a “return value” and then SS,
which is the checksum of the four return value (DDDD) characters:
(stx)DDDDSS(ack)
If the checksum for a command or query is not correct the temperature controller will respond with four upper case X’s and
then 60, which is the checksum of these four X’s:
(stx)XXXX60 (ack)
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