APS SS205-V4+ LV - Printer Driving Timing

Advanced Printing Systems
SS205-V4+ LV - Technical Manual - Rev B Page 16
4.2. Printer driving timing
Printing is always a compromise between 3 parameters:
 Paper feed speed (function of voltage)
 Head activation time (function of voltage, TPH temperature, printing cycle)
 Maximum peak current available (function of voltage and max number of dots simultaneously
activated)
For a given voltage, and a maximum current available, it is easy to determine the maximum paper feed
speed (MaxPFS), as indicated on the above chart. Then, if the two others parameters are not limiting this
speed, it will also be the printing speed (MaxPS).
MaxPFS gives a time (by inverting) called SLT (scanning line time). In this time, the head must be
activated. If this time is not long enough, MaxPS will be subsequently affected.
Then, the way of driving the head is a critical point in the thermal printing application.
A common way to limit the current in the head is to use dynamic division method.
For this, it is necessary to divide data to the head dynamically, by software counting of actual number of
“black” dots. This number of black dots has to be divided by the maximum dot value (64 dots
simultaneously). Software will fill remaining dots with “0” and activates the strobes line. Doing so,
activation will be always done with maximum number of black dots allowed, optimizing number of times
the head needs to be activated. Printing standard text, the average number of black dots is usually less than
64 and sometimes can reach 128.
Example: at 5 V with the SS205-V4-LV, the strobe activation time is, as a quick estimation, 3.85ms*(
2
6.3
/
2
5
) = 2.00 ms. Max current requested to fire up to 64 dots simultaneously is ~2.3A. Max Paper
Feed Speed (MaxPFS) is 35 mm/s.
If the dot line is not full, the number of strobes pulses can be limited at the number of black dots
divided by the maximum number of dots (DOTSmax).
If the maximum current available for the head is 1.9A, DOTSmax to be simultaneously activated will
be Imax / Idot (Current per dot) where Idot=VH / Rdot.
So, using VH = 5V and Rdot = 136 (116 for dot + 20 for the dots drivers, Rcom omitted), we
find:
DOTSmax=1.9/(5/136) = 51 dots (Lower or equal to 64 max simultaneously activated).
So, for a full black line, the maximum number of strobe pulses per line will be 384/51 = 8 pulses. Thus
the total heating time for those pulses will be: SLT = 8 (pulses/line)*2.00(ms/pulse) = 16 ms/line.
Then MaxPS will be 0.125(mm/line)/20(ms/line) = 7.8mm/s (< MaxPFS) as real print speed.
If dotline has 102 max black dots, number of Pulses will be: 102 / 51 = 2, giving SLT = 4 ms/line.
Then MaxPS will be 0.125 / 4 = 31.3mm/s (< MaxPFS) as real print speed.
If dotline has 51 max black dots, number of Pulses will be: 51 / 51 = 1, giving SLT = 2 ms/line.
Then MaxPS will be 0.125 / 2 = 62.6mm/s (>MaxPFS). So, real print speed will be 35 mm/s.
Print speed can be dynamically adjusted, depending on the dot line to be printed.