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1.
Initially bit 7 is put into the B-register.
This will be used to test whether a whole
character has been received.
2.
The NESl command tests to see if the receive flag is set.
If
the flag is not set, the
BZE command branches back to test the flag again.
3.
If
the flag is set, the bit contained in the data buffer is shifted into position 17 of the
B-register.
4.
If
the B-register is even, control is transferred back to get the next bit.
If
the B-
register is odd, meaning the initial bit set in B has reached position 1, the even test
fails and the program continues with the next instruction.
5.
The DEF5 instruction resets the receive clock.
The next example is one method which might be used to transmit one character onto a trans-
mission line via a bit buffer without using the SCN instruction.
Location
Instruction
Symbol
03720
02400
$NCHAR
03720
603120
03721
022002
XMIT
03722
121721
03723
060401
03724
042404
03725
131721
OPR
0£erand
ORG
2000
EQU
1280
LDB
$NCHAR
NES
2
BZE
*-1
TRA
B,T
SRl
B,B
BNZ
XMIT
Remarks
LOAD CHARACTER FROM TABLE
TRANSMIT FLAG SET
NO, GO BACK
TRANSFER BIT TO TRANSMIT DATA DRIVERS
SHIFT B-REGISTER RIGHT ONE
WHOLE CHARACTER NOT OUT, GO BACK
1.
The character to be transmitted is put into the B-register.
2.
The transmit flag is tested to see
if it
is set.
3.
When the flag sets the low order bit of Bis sent to the transmit buffer.
4.
Bits shifted right 1 place and tested for zero.
If
B is non-zero, control is transferred
back to transmit next bit.
When B becomes zero, the BNZ test fails and the program
goes on to execute the next instruction.
The next two examples show how to receive a character and transmit a character using Hard-
ware Scan (SCN).
It should be noted these are examples and do not necessarily show the way
they will be written in the operating programs.
[ID&IJ&~~IJCJ ~@------------
V-19
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