Radio Shack TRS-80 Service Manual page 25

LINE PRINTER AND REAL TIME CLOCK
ADJUSTMENTS AND JUMPER POSITIONS
The jumper positions vary between 4K and 16K RAM
models. Jumpers should be installed as described below.
The LS273 (U941latches Ihe data written 10 the Line Printer
and the one-shot jU93) provides the dala strobe, The printer
status lines are buffered through the LS244 (U95). A full
eight bits of data are buffered
so
thllt the Line Printer inter·
face could be used
rOI
any g.enenl purpose paralJ.e1 interface.
4K RAM
16K RAM
U
10
T, N
10
P, GG
to
FF
StoT.RtoP,EEtoH
Thc Real Time Clock circuit provides either
II
30 Hz or 25 Hz
interrupt 10 lhe CPU if enabled. VDRV is divided by IWO al
U83 (LS74) and latched in Ihe other section of US3. This
generatllS an inlcrrupt through LS38 (USB} and the $tatus
can be read by strobing U84 {LS367J with RDINTSTATUS',
To clear Ihe inlerrupl one mU$l Strobe RTCIN·.
lOBUS
The 10 Bus supportS all Ihe signals necessary to implement a
device comp;:rtible to the Z-SO's 10 Structure. The L$245
(U10ll buffers the della in both directions
and
is enabled by
ENEXTIO. The direction is determined by the L$244 (Ul021
and enabled by ENEXTIO. The control lines necessary for 10
operation are buffered by Ihe LS367 (U 103) and enabled
also by ENEXTIO. EXlernal 10 wails and interrupu are also
supported by the 10 Bus via US7 and U88. Refer 10 lhe
de·
tailed 10 Bus description and timing diagram.
18
The CRT positioning jumpers also very between the 50Hz
and 60Hz models, Jumpers should be installed as dcscrlbed
below. The Vertical and HoriZOf'Ital Position jumpers are
determined by the factory and depend on Ihe frame rate and
Ihe monilor used.
The standard CRT jumper configurations for a 60 Hz com·
puter are:
Vertical Frame Rate:
C to B
Vertical Position Adjustment:
o
10 E
K to L
J to H
Horizonlal Position Adjustment:
V to W
CC to BB
The standard jumper configurations for a 50 Hz computer
aro as follows:
Vertical Frame Rate:
A to B
Vertical Position Adjustment:
D 10 E
M to L
G
to H
Horiwnlal Position Adjustment:
V to W
CC to BB
There norma1ty should be no need to change these jumper
positions, however on some units the display may appear
too fer to the left. If this should occur, change the V 10 W
jumper plug 10 W 10 X. This moves the display one dlaracter
position to the right within the raster.