Texas Instruments 990 User Manual page 93

Applications Programming
Bytes 10 and 11 contain the applications
program~s
output
character count for write operations. This is the
number of bytes (characters) to be transferred from
the application
program~s
data buffer to the
communications buffer. For read operations, the
number of bytes sent to the application program~s
data buffer is returned to this location.
Bytes 12 and 13 are reserved for the communications package.
Bytes 14 and 15 contain the DID of the station or program
for write calls to which output data is addressed.
This word is formatted in four half-byte binary
coded decimal (BCD) values. Applications programs
that send data to secondary programs or remote
devices must furnish the DID for the data to be
routed to the proper location. On read calls, the
SVC returns the network ID of the remote sender in
these bytes. This word is therefore always the
network ID of the remote station or program.
Bytes 16 and 17 contain the SID of the calling station or
program. This word is formatted in four half-byte
BCD values. The SVC supplies the SID if the
applications program makes the call with zero (0)
in these bytes. These bytes always identify the
caller~s
SID, whether the call is a read or
write/send call.
Note that available network IDs in bytes 14 through 17 are
expressed as BCD numbers in the range of 0100 through 9999.
However, they must be supplied in applications programs in
hexadecimal format. For example, a network ID of 0431 would be
entered in the program as >0431. The characters A through Fare
invalid for all network IDs. The IDs 0000 to 0099 are reserved
for the communications package software.
The following paragraphs describe each of the input/output
opera tions.
4.2.1.2 Write/Send Data - Opcode 2.
An SVC >4D, I/O opcode 2, transfers one block of data in the
applications program at the source from its output data buffer to
the communications output buffer for subsequent transmission to
the DID. The DID may be an applications program or a remote
device. Control returns to the applications program when the
data transfer to the communications output buffer is complete.
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