RCA 301 Programmer's Reference Manual page 130

If the printer is an unbuffered device, the print table and the data line are accessed in the High-Speed Memory.
As the print drum revolves, the printer sends signals to the Computer indicating which character on the print drum
will next be in the print position. These printer signals are used to develop an address for an automatic table lookup
operation. This operation extracts from the Print Table (see Appendix H) the RCA 301 character which is to be
printed next. This character is stored in the Print Register. The diad addressed by the A Register is read out of HSM
and checked for the character stored in the Print Register. The contents of the A Register are increased by two and
the cycle is repeated. Modification of the contents of the A Register and checking of the addressed diad continue
until 120 or 160 consecutive memory locations have been checked. This checking for one character is known as
a "scan."
As each character is checked a one bit is sent to the printer shift register for an equality, a zero bit is sent
for each non-equality. At the completion of a scan the shift register contains a one bit in every position on the
line in which this character is to be printed. The one bits are then used to trigger the corresponding priht hammers.
The contents of the B Register are again transferred to the A Register, a new character is read from the Print Table,
and another scan is begun.
There are two modes of operation, Synchronous and Asynchronous. The Synchronous Mode permits only 47
characters to be printed since a complete line of print and paper advance to the next print line is accomplished during
each revolution of the print drum. The letter "A" is always the first character for which HSM is scanned, and the
period is the last.
The Asynchronous Mode permits all 64 characters on the print drum to be printed (120 or 160 characters per
line) in one full revolution, and paper advance occurs in a portion of the next print drum revolution. Scanning can
proceed with the next character in sequence on the print drum at the conclusion of the paper advance function.
Appendix H shows print table containing RCA 301 characters. The code (17)
certain characters on the print drum. An inhibited character may be used to specify a non-represented RCA 301
character by substituting the RCA 301 character's bit configuration into the proper print table position. For example,
the EB symbol may be printed as "-4-" by inserting (72)
Final Register Contents
(A), = Address of location one to the right of the last character printed if printing is done, otherwise it is (A),.
(B) = (B), except that B
t
Timing
Print and paper advance of one line takes 60 ms. in the Synchronous Mode or 76 ms. in the Asynchronous Mode.
When more than one line is to be advanced, paper advancing is done at the rate of 6.67 ms. per line after the first line.
Example
Instruction:
Operation
B
HSM before and after Instruction is Executed:
(First Character Printed)
1201
1200
2
1
Paper is advanced one line after the printing of the line.
Final Register Contents:
(A), = 1320
(B) = 1200
f
Time:
76 ms.
X-4
in location 9933, or 1933, or Z933 of the print table.
s
will be set to zero.
3
N
1
(Last Character Printed)
^
is used to inhibit printing of
s
A Address
0000
B Address
1201