Model 574 Programmable Counting System (PCS)
290 - GSE Scale Systems
indicator then the transmission must include a checksum
and a mechanism to re-attempt a transmission in case of
errors in order to be PTB approved.
Several different styles of checksums may now be
calculated by the indicator to help insure the integrity of
the transmitted data. One of these new checksum
calculation methods matches that used by Epson in a
protocol commonly used in Europe. Together with
capabilities of the Input Interpreter (P900), the 550
indicator may now be used with these Epson printers,
insuring correct data transfer by re-sending the
transmission if the required acknowledge is not received.
A data checksum calculation consists of three
commands:
1. Initialize and begin calculating a specific type
of checksum starting with the next transmitted
character.
2. Stop calculating the checksum (optional).
Required only when the checksum is not to be
transmitted until after some additional
characters are transmitted.
3. Transmit the checksum. Since most supported
checksums are two-byte, there are two
commands, one for most significant byte (msb)
first and one for least significant byte (lsb) first.
Both bytes will be transmitted in succession.
For single byte checksums, either command
may be issued.
Several new codes have been defined which allow these
commands to be embedded at the proper locations in a
custom transmit or to be done at a particular time within
a macro. Similar to the way a carriage return/line feed
combination may be programmed into a custom transmit
setup by entering .256, the codes for the checksum may
be entered as shown in table 51.
To allow one these checksum commands to be issued
directly from a macro, use the appropriate code listed
above along with the "send control code" command, "%
&". For example use 306%& to begin a LRCC-8
checksum.
Refer to app note 1 for an example of how to interface an
Epson printer with the 550 indicator using checksums.
(TO FOLLOW)
Note that this feature only allows the transmitting of
checksums, not the receiving of checksummed data.
33.5 Printer Interface Example
GSE Model 550 to Epson Printer
Implementing Epson "BT-90" Block Transfer
(commonly used in Europe)
Checksum Code Function Description
.300
stop Stop calculating
the checksum but
do not transmit
yet.
.301
CCITT Cyclical
redundancy
check, per .....
.302
SDLC
.303
CRC-16
.304
CRC-12
.305
LRCC-16
.306
LRCC-8 8 bit CRC, used
by Epson
.307
XMODEM
.308
Sum 16 2 byte additive
checksum
.309
Sum-8 1 byte additive
checksum
.310
Send Checksum Transmit
checksum
sending LSB first
.311
Send Checksum Transmit
checksum
sending MSB
first
Table 51, Checksum Codes
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