Epson PX-8 Technical Manual page 157

REV.-A
3.1.7 Read/Write Control Section
The read/write control is located in the lower half of the MMC board and consists of a read and
write circuit. However, most of the section is actually occupied by the read circuit operational am-
plifier.
3.1.7.1 Write Circuit (Erase and Write)
Magnetic tape write is accomplished using leakage flux as illustrated above. A logical value of 0
or 1 can be written on reversing tape by reversing the direction of the flux; i.e., the direction of the
current through the write head winding.
Write
This circuit reverses the write current direction
by means of charging and discharging an elec-
trolytic capacitor. Thus, the current waveform
theoretically looks as shown in Fig. 3-14.
current
State
Magnetized
polarities
on tape
Discharge
Fig. 3-14 Tape Write Current Waveform
Writing a value is ultimately generating a magnetic polarity determined by the write current direc-
tion. In order to eliminate interference which may be caused by data patterns previously written
(magnetized) on the tape, the erase head is always activated during write operation initiated by
the ERAH signal. New data, then will not be affected by previously magnetized polarities and the
tape can be uniformly magnetized as new data is written Fig. 3-15 conceptually illustrates this
operation, including the positional relationship between the read/write and erase heads.
Q)
c.
ctI
-
U
.'::;
Q)
c
OJ
ctI
~
Tape feed direction
N N N N N N N N N N
R/W coil
N S N S S N
n~rn~r
S N S N N S
\ I
Newly written data
s s s s s s s s s
S
\ I
--t--
~
_ _
l~
Erased area
RPH+ RPH-
Fig.3.15
3-11
S S N N S
~rrlI
N N S S N
~
-i--
Previous data
E+ E-
/