Barco ACTAS 2 Owner's Manual page 53

Appendix B : Serial communication
There are two formats of data transfers :
K
A short one : called "record"
K
A long one : called "block" (this format is currently
not used for ACTAS purposes)
A record of the short format consists of 7 bytes :
ADR
CMD
DAT1
DAT2
DAT3
DAT4
CHKS
CMD is the command given to the slave with the address
ADR . DAT1-4 can contain additional information (ex-
plained later in this text), however, this information is not
always used. It is advised to set all non-used data bytes
to 0 (zero). CHKS is:
( ADR + CMD + DAT1 + DAT2 + DAT3 + DAT4 ) MOD 256
The transmission of the record on the serial line will be:
STX OFFS ADRe CMDe DAT1e DAT2e DAT3e DAT4e CHKSe
The long format 'block' consists of a record of 9 bytes
followed by either 208, 224, 240 or other data bytes plus
their checksum.
Slave type
800, 801,1100, 1101, 1600, 500
701
808, 1208, 1209
ACTAS
The transmission will be :
STX OFFS ADRe CMDe DAT1e DAT2e DAT3e DAT4e CHKSe
D0e D1e ... D207e CHKSBe
with
Die = Di + OFFS
CHKSBe = ( CHKSB + OFFS ) MOD 256
CHKSB = ( SUM (i = 0 to 207) Di ) MOD 256
Bytes expected back from slave
Two Main types of slave response are possible :
K
No answer data bytes required;
K
Answer data bytes required.
In any case, at least a handshake byte has to be
returned.
Note that the slave only transmits upon request from the
master.
Handshake byte :
Because only the signals RxD and TxD are used for the
communication, there is no way to detect if there is a
slave listening or if there is one listening whether it
receives the data correctly. Therefore, the communica-
B-2
Slave address
Command
Data byte 1
Data byte 2
Data byte 3
Data byte 4
Check sum
Block length
208
224
240
- (reserved for
future use)
tion protocol expects a standard ASCII byte ACK( 06H)
or NAK (15H) from the slave.
ACK
NCK
It is very important that when the master asks the slave
for information that the master is ready to receive
immediately the data from the slave. Otherwise, data
loss or overruns could occur, or after a "time out", the
master could think that there is no slave responding.
Data bytes :
If answer data is required, this can be either just a record,
or a record followed by a block.
No answer data from slave required
For the record format
For the record format (short) the slave returns an ACK
byte if it has received the data correctly or a NAK byte if
the received data is not correct. If there is no slave
responding, then no byte is returned.
Master to slave :
RECORD
Slave to master :
ACK or NAK
For the block format
For the block format (long) the slave returns an ACK (or
NAK ) after the first part (the record) and a second ACK
(or NAK ) after the second part.
Master to slave :
RECORD
Slave to master :
ACK or NAK
Answer data from slave required
For the record format
Suppose the master asks for information to the slave
and the slave returns the information in the record
format, the bytes transmitted through the RS232 com-
munication will be :
K
Record from master to slave :
STX OFFS ADRPe CMDe Dat1e Dat2e Dat3e Dat4e CHKSe
K
Record from slave to master :
ADRP CMD Dat1b Dat2b Dat3b Dat4b CHKSb
where
ADRP = ADRPe - OFFS
CMD = CMDe - OFFS
acknowledge
not acknowledge
BLOCK
ACK or NAK
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