Control system for timing hammers of impact printers

A hammer timing control system for a line printer has a register for storing a digital delay value related to the actual flight time of a controlled print hammer. A delay counter activated by an initiate fire pulse counts timing pulses until the count equals the delay value in the register. A comparator generates a fire signal when the count equals the delay value in the register. The counter continues counting pulses to a second count whereupon the fire signal is terminated. The second count is either decoded or compared with a predetermined quantity in a second register to terminate the fire signal. The counter may also be an up/down counter which counts up to delay and down to terminate the fire signal. Delay values can be stored in an external memory device such as a magnetic disk for transfer to the individual registers for each of the print hammers.

FIELD OF THE INVENTION 
This invention relates to high speed printers and particularly to a control 
system for accomplishing improved registration of printed characters in an 
electromechanical printer system. 
BACKGROUND OF THE INVENTION 
In high speed on-the-fly line printers a plurality of print hammers usually 
arranged in a row are selectively operated to strike the type faces on a 
constantly moving type carrier. The type carrier may be a revolving 
flexible band, belt, chain or train or a rotating drum. The print hammers 
are generally operated electromechanically preferably using 
electromagnetic actuators including an armature which when the 
electromagnet is energized, i.e. fired, propels an impact element or 
hammer from a rest position to the point of impact. Commonly, the armature 
stroke is stopped, i.e. seals, before impact while the hammer element 
continues in free flight to the point of impact. At the instant of impact, 
the hammer rebounds to be restored to the rest position where after a 
brief period of settling comes to rest ready to be fired. 
Good registration of the printed characters requires that hammers be 
controlled so that impact occurs at the exact time that the desired 
characters become aligned with the selected print hammer/print position. 
It is further desirable to be able to terminate the energization of the 
electromagnet at or slightly after the time the armature seals, thereby 
saving energy and to be able to accommodate for the period during which 
hammers are settling in preparation for repeat firing. It is also 
desirable that the flight times be easily changed to accommodate variances 
in hammer operating characteristics during a relatively extended use 
period and that these changes be made without the need for altering 
control circuitry. 
Various control schemes have been devised for operating print hammers to 
compensate or adjust for variation in the actual flight times of the print 
hammers due to variances in printer operating characteristics. Basically 
these control schemes introduce variable delay circuits into the hammer 
fire circuitry. While some of these systems may largely dispense with the 
arduous and time consuming task of manually adjusting hammer flight time, 
they are essentially inadequate for achieving reliable precision hammer 
flight control required for very high printing speeds, e.g. where the type 
carrier speeds greatly exceed 300 inches per second. Also they lack the 
capability to be easily and readily adapted to control the time for 
terminating the energization of the electromagnet and/or to make 
accommodation for the settling time of the hammers before they are again 
fired. Most prior art control schemes require complex timing controls 
and/or require changes in circuitry or circuit components to make the 
adjustments which compensate for changes in the operating characteristics. 
BACKGROUND ART 
U.S. Pat. No. 3,183,830 issued May 18, 1965 to D. M. Fisher et al discloses 
a print registration control in which misregistration of printed 
characters is corrected by delaying the individual signals applied to the 
respective hammer operating solenoids. For this purpose, a variable 
one-shot circuit is provided for delaying the operation of a fixed delay 
one-shot circuit which controls the energization of the solenoid winding 
for a fixed time interval. A variable resistor which determines the 
discharge time of a capacitor is adjusted to alter the delay period of the 
variable one-shot circuit so that all printed characters in a line of 
print are in registration. 
U.S. Pat. No. 3,872,788 issued Mar. 25, 1975 to G. A. Palombo describes a 
closed loop system wherein a variable delay circuit is introduced into the 
command input to the hammer. The variable delay circuit is a counter 
presettable to a predetermined delay count condition stored in a storage 
counter. The delay circuit counter is reset after it has achieved a full 
count condition to the initial desired delay count condition by a feed 
back pulse from a hammer fire latch which effects the transfer of the 
stored count condition from the storage counter to the delay counter. 
Alternatively, the delay counter continues to count pulses from a clock 
controlled pulse generator after the delay counter has initiated the 
hammer firing and until such time as the delay counter again reaches the 
initial preset count condition. A hammer initiated fire pulse of a time 
duration equal to the full count of the delay hammer maintains the pulse 
generator on until the delay counter reaches the initial preset count 
condition. The hammer pulse provided by a monostable circuit has a fixed 
time duration. 
U.S. Pat. No. 4,286,516 issued Sept. 1, 1981 to H. Wertanen describes an 
electronic control for timing hammers which utilize digital logic 
circuitry which varies the timing of pulses that drive hammers in an 
impact printer. The control controls the timing of the firing pulse to 
each hammer by retarding it or advancing it from a nominal built-in time 
delay to compensate for differences in spacing between printed columns. 
Variations are made in the electrical circuitry to adjust the spacing. The 
electronic control includes a field alterable preprogrammed read only 
memory consisting of driver/decoding circuits connectable for feeding 
through a plurality of settable switches. The settable switches produce 
weighted on signals which in combination with the counter controlled 
multiplexor control the timing of firing pulses from the multiplexor to 
selected print hammers. Adjustment in spacing is made by changing the 
setting of the switches and hence the weighting of the on signal. Drivers 
for the print hammer consist of one-shot multivibrators driving Darlington 
circuit devices to generate fixed width drive pulses. 
SUMMARY OF THE INVENTION 
The electronic control of the present invention provides for automatic 
flight time compensation and other print hammer controls without complex 
timing arrangements and without the requirement for making circuit changes 
to make adjustments made necessary by changes in the operating 
characteristics of the print hammers. Other advantages may also be 
obtained from the invention. 
Basically, the hammer timing control system of the present invention 
comprises register means for continually storing a digital delay fire 
quantity representative of the acutal flight time of a controlled hammer, 
clock means for producing a continuous stream of timing pulses, a delay 
fire counter means operable in response to an initiate fire signal for 
counting timing pulses produced by the clock means and hammer fire circuit 
means for producing a fire hammer signal in response to a count condition 
in the counter means which corresponds with the delay fire quantity in the 
register means. The control system further provides circuit means 
responsive to a second count condition of the counter means for 
controlling the hammer fire circuit means to terminate the fire hammer 
signal at the predetermined second count condition of the counter means. 
In one embodiment, the system includes a decode connected to the output of 
the counter means for detecting the second count condition and for 
generating a fire terminate signal. In another embodiment, the control 
system provides a second register which stores a terminate fire quantity 
representing a fixed time after the initiate fire signal and prior to 
impact. The fire control system further provides circuit means responsive 
to a second count condition of the counter means corresponding with the 
terminate fire quantity in the second register means for terminating the 
fire hammer signal. In the preferred embodiment, the control system 
utilizes a signal comparison circuit for comparing the count condition of 
the counter means alternatively with the delay fire quantity in the first 
register means and with the terminate fire quantity in the second register 
means. Gating circuits operable by separate control signals connects the 
registers to the comparison circuits.

DETAILED DESCRIPTION OF THE INVENTION 
As seen in FIG. 1, a print hammer mechanism for a single print position of 
a high speed printer and suitable for practicing the invention includes an 
actuator 10 consisting of coils 11 on poles 12 of stationary magnetic core 
13. Coil 11 when energized by current pulses I.sub.HD from hammer driver 
circuit 14 drives an armature 15 pivoted at 16. The mechanical energy 
induced in armature 15 is coupled to hammer element 17 by means of pushrod 
18 supported in guideway 19 of block member 20. In the non-energized 
condition of coil 11, hammer 17 and pushrod 18 are loaded by bias springs 
21 and 22 to rest against the backstop of core 13. When coil 11 is 
energized, armature 15 overcomes the bias force of springs 21 and 22 and 
drives hammer 17 until armature 15 seals, i.e. is stopped and held against 
poles 12. When armature 15 seals, hammer 17 has received all available 
energy and therefore continues moving under its own momentum until impact 
forcing paper 23 and ribbon 24 against type face 25 of the moving print 
band 26 which is backed by stationary platen 27. After impact with type 
face 25, hammer 17 rebounds from the paper 23 and type face 25, moving 
armature 15 from its sealed position to the rest or backstop position. 
Armature 15 and hammer 17 bounce around the rest position until settling 
is finally attained. The period from the instant the hammer driver 14 is 
activated to the instant when hammer 17 is in the steady state rest 
position is referred to as the hammer-busy period T.sub.HB. It is 
sometimes called the hammer settle out time and is the limiting factor in 
determining the maximum fire repetition rate. Firing a hammer 17 before it 
has settled would result in erratic variations in flight time and impact 
force. 
While a single print hammer mechanism is described in FIG. 1 it is 
understood that a line printer in which the invention is practiced would 
utilize a plurality of such print mechanisms; for example, one for each of 
a plurality of print positions located along a print line. A multiple 
print hammer assembly for a line printer which may be employed for the 
present invention can be seen and understood more fully by reference to 
U.S. Pat. No. 3,241,480 issued Mar. 22, 1966 to J. M. Cunningham. 
The timing of the operation of the print mechanism of FIG. 1 is understood 
further by reference to FIG. 2. As shown in the timing circuit chart of 
FIG. 2, the real hammer flight time T.sub.F is defined as the elapsed time 
from the instant hammer driver 14 is activated by a fire control pulse 
I.sub.HD to the instant impact occurs. The seal time T.sub.SL is the 
interval between the instant driver 14 is activated by the fire control 
pulse I.sub.HD and the instant armature 15 seals against poles 12 of core 
13. During the interval T.sub.F -T.sub.SL hammer 17 is in free flight. 
Since no additional energy can be transferred to hammer 17 once armature 
15 seals against poles 12 driver 14 need not continue to energize coil 11 
and the pulse I may be terminated. In other words, hammer driver 14 need 
remain active only for the period T.sub.ON which is in accorance with the 
preferred embodiment of this invention is equal to or greater than the 
period T.sub.SL over the operating range of hammer 17. This is defined by 
the following expression T.sub.ON =T.sub.F -T.DELTA. where T.DELTA. is the 
minimum free flight time and driver 14 turns off no sooner than armature 
seal time but before impact time under normal operating conditions. 
Further as seen in FIG. 2, hammer driver 14 is turned on in accordance 
with this invention at T.sub.1 which occurs at some variable delay time 
T.sub.DF after initiate fire time at T.sub.0. 
In the preferred manner of practicing the invention, the effective hammer 
flight time T.sub.EF as shown in FIG. 2 is a constant for all print 
hammers. Hammer driver on time T.sub.1 occurs after a delay interval 
T.sub.DF which is variable dependent on the actual flight time 
characteristics of each hammer. Terminate fire time T.sub.3 which can vary 
for each hammer depending on the operating characteristics thereof always 
occurs at or after the hammer seals at T.sub.2 but before impact at 
T.sub.4. 
In a printer control system for a group of a set of print hammers, as seen 
in FIG. 3, coil 11 is connected to be energized by hammer driver circuits 
14. Each hammer driver circuit 14 is connected to an individual fire 
control circuit 30 which functions to control the turn-on and turn-off 
times of driver circuits 14 which in turn controls the drive current. 
Current is supplied to coil 11 for operating the individual print hammers. 
Timing pulses generated by a suitable timing source such as a free running 
clock which may be part of a control system are supplied through clock bus 
31 which has input connections 32 to the flight control circuits 30. 
Hammer selection is obtained through hammer address bus 33 connected to 
the interconnected address decode 34 and the 1 of 6 decode circuitry 35 to 
hammer select bus 36 having a second input 37 to the flight control 
circuits 30. Flight control data for timing the turn-on time and terminate 
fire data for controlling the turn-off time of the individual hammer 
driver circuits are provided on data bus 38 with inputs 39 to the flight 
control circuits 30. Various control signals are supplied to the flight 
time controls 30 via bus 40 with inputs 41. DAC register 42 has an input 
43 connected to data bus 38 and a control connection 44 to control bus 
40. The output 45 from DAC register 42 is connected to the hammer driver 
circuits 14. DAC register 42 functions to convert digital data to analog 
signals for adjusting the hammer driver circuits 14 to change the current 
levels and hence the energy supplied by the hammer driver circuits to coil 
11. Such energy level changes are desirable where printing is to be done 
on print media having different thicknesses such as 1-12 layer paper forms 
of the type used for recording multiple copies of business and/or 
scientific data. A driver circuit suitable for use with this invention 
which includes DAC register for adjusting the current, i.e. energy levels, 
of the driver circuit is more fully described in the co-pending 
application of R. W. Arnold and D. W. Skinner, Ser. No. 274,848, filed 
June 18, 1981. Energy level selection is made through a multiple position 
forms switch or the like located on the printer and operated at the time 
paper is loaded into the printer whereupon the printer controls which may 
include a microprocessor which monitors the forms switch loads the energy 
level data form data buss 38 on connection 43 to DAC register 42 
concurrently with the generation of a load signal LD DAC R applied to 
connection 44 of control bus 40. 
The control system may also include a status multiplexor 46 connected to 
address bus 36 and to the hammer driver circuits 14 for the purpose of 
checking the condition of the hammer driver circuits as a group or 
individually. 
Other control signals applied to control bus 40 also from external controls 
which might include a microprocessor or the like include the following: 
1. --LOAD DFR-- This signal is used for loading flight control data on data 
bus 38 into the flight control circuits 30 addressed by hammer decode 
logic 35. 
2. --LOAD TFR-- This signal is used for loading the terminate fire data on 
bus 38 into flight control circuits 30 addressed by the hammer decode 
logic 35. 
3. --INIT. FIRE-- This signal is used for initiating flight control circuit 
operation which compensates for the different flight times of the print 
hammers as determined by the flight control data and which ultimately 
generates the fire ham. control to hammer driver circuits 14. The INIT. 
FIRE signal is preferably generated by external controls which compare the 
contents of a print line data storage device with a type carrier image 
storage device in synchronism with the movement of the type carrier and 
generates the signal when a comparison occurs. This signal is timed to be 
generated on control bus 40 to the flight time control circuits 30 as so 
to always occur at a fixed time T.sub.EF prior to the impact of the print 
hammers. 
Other control signals which may be applied to control bus 40 by external 
controls will be discussed hereinafter. 
In the preferred embodiment in which the invention is practiced, each 
flight control circuit 30, as shown in greater detail in FIG. 4, comprises 
delay fire register 50, counter 51 and comparator circuit 52 for each 
print hammer. A terminate fire register 53 which preferably is shared with 
other flight control circuits for controlling all or a group of print 
hammers is also provided. Delay fire register 50 stores a delay value for 
delaying the time when the print hammer is to be fired, that is when the 
hammer driver is turned on for energizing coil 11. Terminate register 53 
stores a time value which controls when hammer firing is terminted, that 
is when driver circuit 14 is turned off ending the supply of current to 
coil 11. Counter 51 functions to time both events. 
The time delay value stored in delay register 50 is an 8 bit binary number 
loaded from data bus 38 by the printer system control signal -LOAD DFR 
applied on line 54 of control bus 40 along with the hammer address on 
address bus 33 though address decode logic 34 and 35 through bus 36 and 
input 37. The terminate value stored in terminate register 53 is also 
loaded from data bus 38 by the system control signal -LOAD TFR on line 55 
from control bus 40. Either the time delay or the terminate value can 
readily be changed to adjust for new or variable operating conditions by 
supplying new values on data bus 38 along with address data on bus 33 from 
any external source under external system control which may be a 
microprocessor using microcode or other programming. Where the delay and 
terminate values remain valid over an extended period of operation, they 
can remain in their respective registers after loading without change. 
Altenatively, should some of or all of the print hammers need adjustment, 
a single, several or all of the delay values can be easily adjusted by 
loading new values directly into the desired registers. Since the hammer 
flight time T.sub.F for the various hammers is a variable parameter due to 
various factors inherent to the structure of the electomagnets and the 
hammer mechanisms, the delay values stored in register 50 are likewise 
varied. The real flight time T.sub.F is a measurable quantity and can be 
expressed as a digital value. Known devices for measuring flight time use 
transducers such as an impact bar located at the position normally 
occupied by the type carrier. The controls for determining the delay value 
of a given hammer count timing pulses from a clock from the instant a 
hammer driver is turned on until an impact signal is generated by the 
transducer. The process may be repeated several times for each hammer. The 
number of timing pulses is then averaged and compared with a quantity 
representing a suitable design standard and any differences calculated for 
use as a time delay value. The delay values for all the print hammers are 
similarly determined and then stored. Delay values can be determined at 
the time of printer manufacture and recorded on a suitable permanent 
record such as a magnetic disk or tape which can be supplied with the 
printer. This record can, in accordance with this invention, be used to 
precondition the printer controls in advance of beginning the printing as 
part of the startup procedures. That is, the delay values recorded on the 
permanent record are read into the delay registers 53 as previously 
described. Because the impact transducers cannot be located at the precise 
impact position of the type carriers, some anomalies may exist in the 
delay values. The present invention permits individual or multiple 
adjustment of the delay values applied to the delay register 53 by the 
recorded values. Additionally, after prolonged use where accumulation of 
dirt, aging or other conditions occur new delay values would be required. 
New sets of values may be again obtained by actual measurement and stored 
as in the case of the original values. 
Counter 51 is a multiple stage binary counter preferably having wraparound 
capability. Counter 51 is connected by AND circuit to the T.sub.2 and 0.4 
MH lines of clock bus 31 and the Q output of start latch 57. The S input 
of start latch 57 is connected to the hammer selection controls of the 
printer system which generates a compare or INIT. FIRE signal which 
enables counter 51 for counting timing pulses gated through AND circuit 56 
at T.sub.2 time. Counter 51 has a reset input connection through OR 
circuit 58 for receiving a reset pulse RSTDFC which clears or initializes 
the count in counter 51 at the beginning of each print operation. Counter 
51 has a multi-bit output connection 59 for applying the count condition 
signal to input B for comparison by comparator 52. 
Comparator 52 also has a multi-bit input A connected to OR circuit 60. The 
delay fire register 50 is connected through AND circuit 61 to one input of 
OR circuit 60. Terminate register 53 is connected through AND circuit 62 
to the other input of OR circuit 60. A set mode count signal on input 63 
to AND circuit 61 gates the delay value stored in delay register 50 
through OR circuit to input A of comparator 52 for comparison with the 
count condition on connection 59 to input B. A RST mode count signal on 
line 64 to an input of AND circuit 62 gates the terminate time value in 
terminate fire register 53 through OR circuit 60 to input A of comparator 
52 for comparison with a second count condition appearing on connection 59 
to input B. 
Comparator 52 has an output line 65 connected to and input of AND circuits 
66 and 67 connected respectively to the S and R inputs of hammer latch 68. 
Set mode count and T.sub.7 clock signals applied to AND circuit 66 gate a 
fire equal compare signal on line 65 when the count condition of counter 
51 equals the delay value of register 50 to set hammer latch 68. This 
produces a Fire HAM. signal at the Q terminal of latch 68 turning on 
driver circuit 14. RST mode count signal and a T.sub.9 clock pulse from 
clock bus 31 produce a terminate equal compare signal through AND circuit 
67 when the count condition of counter 51 after one or more wraparound 
operations equal the terminate value from register 63 to reset hammer 
latch 68. This causes hammer latch 68 to terminate the Fire HAM. signal at 
the Q output thereby turning off the driver circuit 14. Line 69 connecting 
the output of AND circuit 67 to OR circuit 58 and the reset input of start 
latch 57 supplies a signal which resets start latch 57 blocking further 
counting operation by counter 51 are resets counter 51 to the initial or 
clear count condition. The control circuit repeats the operation for 
successive printing operations and compensates for different flight times 
of the various hammers in accordance with the delay values specified in 
the delay register 50. 
Terminate fire values in register 53 preferably are designed to shut off 
driver circuit 14 at a fixed time before impact. This value can be the 
same for all hammers. The terminate fire value is also selected in 
accordance with this invention to occur after the armature of the 
electromagnet actuator is stopped and held at the sealed position. 
The delay fire value stored in register 50 can also be used to compensate 
for other operating conditions. It is a specific feature of this invention 
that the delay fire value stored in register 50 also account for the 
flight time changes related to the energy of the hammer as set by the 
adjustment of driver circuits 14 through the operation of DAC register 42. 
In determining the delay values for the various energy levels, the hammers 
are operated with the impact bar installed as previously described at the 
different energy levels and delay values computed accordingly. The set of 
values for each energy level are stored on a recording device such as a 
disk as previously described as part of the startup routine for the 
control system. These values may be stored as a set of tables to be read 
into a random access memory device which is part of the control system for 
later use as needed during the course of printing on different thickness 
forms. 
A specific set of binary delay values for a given hammer for use in a 
printer wherein the type carrier speed is 500 inches per second for 
printing at four different energy levels is as follows: 
EQU 25, 52, 102, 214 
The actual delay time represented by a specific binary value in the 
register is equal to that value times the period of the T.sub.2 clock. 
In addition to compensating for flight time variations of the print hammers 
and the energy level at which they are operated, the delay value stored in 
register 50 may take into account other operating parameters. 
Specifically, one operating parameter which could be readily incorporated 
into the delay value of a given hammer is the time increment required to 
delay the firing of the hammer dependent on its position relative to the 
first print hammer in a row of print hammers in a belt or chain type 
printer. Thus the delay value for a print hammer at print position 45 may 
have an additional .DELTA. time added to the normal flight time delay for 
that hammer to compensate for the flight time variation and for the timing 
of the firing of the hammer relative to the motion of the type belt. A 
specific example for a delay value which includes the added print position 
for a print hammer operating with a type carrier having a velocity of 500 
inches per second is as follows: 
EQU 40, 67, 117, 229 
Thus it will be seen that a very precise control has been provided which is 
very versatile and requires a minimum of time, utilized simple timing 
arrangement and does not require circuit changes to adjust hammers to 
different operating condition with different flight time characteristics. 
FIG. 5 shows a second embodiment having an alternative arrangement which 
also uses a single counter for timing both the delay of the hammer firing 
and the termination of the hammer firing signal. In addition, the counter 
functions to determine the time at which the hammer settle condition has 
occurred. As seen in FIG. 5, delay register 50 has a direct multi-bit 
connection 70 to input A of comparator 52. Counter 71 is a 9 bit counter 
for example having the lower 8 bits applied by connection 72 to the B 
input of comparator 52. The output of comparator 52 is connected by line 
73 to the S input of hammer latch 68 to turn on the hammer driver circuit 
14 when an equal compare signal appears on line 73. Connection 74 connects 
the 9 bits of counter 71 to AND circuit 75 which has a second input 76 and 
an output 77 connected to the R input of hammer latch 68. Connection 79 
connects the 9 bits of counter 71 to AND circuit 79 which has input 80 and 
has an output 81 for connection to external control for recognizing the 
hammer settle condition signal appearing on line 81. 
The circuit arrangement of FIG. 5 operates in substantially the same manner 
as the previous embodiment. Delay values are loaded in delay register 50 
for comparison by comparator 52 with the lower order count condition 
appearing on connection 72 as the counter is advanced by clock pulses 
gated through AND circuit 56 upon the initiation of firing by an INIT. 
FIRE pulse. When the compare equal occurs, a compare equal signal on line 
73 sets latch 68 turning on the driver circuit. Counter 71 continues 
counting timing pulses until the counter is filled whereupon it wraps 
around and continuous counting until it reaches the high order condition. 
At this point a Wrap One signal, generated by a counter full flag, 
appearing at line 76 is grated through AND circuit 75 applying a reset 
signal on line 77 resetting hammer latch 68 and turning off the driver 
circuit 14. The Wrap One signal causes the clock to switch to a lower 
frequency. Timing pulses continue to advance counter 71 at a slower rate 
to the full count condition whereupon it wraps around a second time. A 
Wrap Two signal appearing on line 80 is gated through AND circuit 79 when 
counter 71 reaches the upper level count condition identified as settle 
time. The external control, seeing the hammer settle signal on line 81 can 
then proceed to fire the hammer again. 
In this embodiment, counter 71 times the delay value, the terminate value 
and further provides an indication of the settle time which allows the 
hammer to be refired. 
FIG. 7 shows a third embodiment in which the counter means for timing the 
delay of the hammer fire signal in accordance with the delay value stored 
in register 50 is a bi-directional counter which is controlled to count in 
one direction for timing the delay and in the other direction for timing 
the termination of hammer firing. As seen in FIG. 7, counter 83 is an 
up/down counter having multi-bit output connection 84 to the B input of 
comparator 52 which is also connected at input A to delay register 50. The 
output of comparator 52 is connected by line 85 to the set input of hammer 
latch 68 and when an equal compare signal is generated by comparator 52 on 
line 85 latch 68 sends a fire signal to the driver circuit 14. Hammer 
latch 68 has its R input connected to the zero count line 86 of counter 
83. Count up control is provided by count up latch 87 which is set by the 
coincidence of a set pulse on line 88 and a hammer address input on line 
89 to AND circuit 90 thereby gating timing pulses on line 91 to advance 
counter 83 in the up count direction. Count up latch 87 has its R input 
connected to output line 85 from comparator 52 for resetting by an equal 
compare signal produced by the comparison of the delay value in register 
50 at input A with the count condition of counter 83 appearing on line 84 
at input B. When count up latch 87 is reset, counter pulses on line 91 are 
blocked from advancing counter 83 further. 
Count down control is provided by count down latch 92 which is set by the 
coincidence of the hammer address on line 89 which gates a reset pulse on 
line 93 through AND circuit 94. When set, count down latch 92 gates timing 
pulses to advance counter 83 in a downward direction. When the zero count 
condition is reached, a count equals zero on line 86 resets hammer latch 
68 turning off the driver circuit 14. The count equals zero signal 
appearing on line 95 resets count down latch 92 thereby blocking further 
timing pulses from advancing counter 83. 
This arrangement would have utility where the terminate fire time and delay 
fire times are equivalent where the desired on time of the hammer driver 
exceeds the delay time value. Means may be provided for delaying the 
beginning of the down count. Such means may take various forms but might 
include means for delaying the gating of the reset pulse on line 93 to AND 
gate 94 for setting the count down latch 92. Such count down delay control 
may be part of the control system which might include a microprocessor of 
a printer control and may be in the form of a software or microcode 
control for operating the microprocessor. 
Thus it will be seen that a flight time hammer control has been provided 
which is simplified using counters and registers along with comparator 
circuits which eliminate the need for making circuit changes to 
accommodate various operating parameters and which uses simple timing 
control. The invention in its several embodiments offers versatility in 
controlling the time of firing of a hammer to accommodate for various 
operating conditions including the inherent differences in the flight 
times of the hammer as well as other properties including changes in 
energy level to accommodate print media of different thicknesses and in 
print hammer position in a row of hammers of a line printer. Other 
advantages may be also realized from the invention. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that the foregoing and other changes in form and 
details may be made therein without departing from the spirit and scope of 
the invention.