HP 13255 Technical Information page 23

13255
General Purposp. Asychronous Oata Comm
~odule
13255-91089/21
Rpv AUG-01,-76
3.9.1.1 In
th~
floating conflquration, the transmitter sources current. The
cyrrent
oat~
Is into the CL+ (P2, Pin 4)
termln~l
and out the CL- (P2,
Pin
5)
terminal. When a teletype is the transmitter, the
TTy IN
(P2,
Pin 12) input_should he used instead of the CL+ input. In the non-
floatinq configuration, CL+12 (P2, Pin 1) and CL+ are connected, and
CT,· is connp.cted to OTounrl. (A passive transmitter should be used with
this cOnfIQuration.) The current path is from +12V into CL+ and
ou~
of
Cl,· to qround. The
tTansmitt~r
can then short the CL+ and CL- termi-
nals and make the path from +12V to qround directly.
3.g.1.?' The output current of the
5082-43~2
opto-lsolator
(Ult)
is converted to
a voltaqe with a
2.2K
r~sistor
and sensed wJth a 74LS132
(Ut9).
From
a distortion standpOint, the
~dea)
O-to-20 rnA or.20
rnA
to
0
tranSition
sense level is
10
m~.
However, nOise considerations dictate that some
hysteresis should be incorporated into the desiqn. The current transfer
ratio of the opto-isolator and the thresholds of the 74LS132 are the
primary factors in determiD1nq the current thresholds of the receiver.
The O-to-20
m~
transitIon 15 sensed between 8.2 mA and 13.4 mAe The
20 mA to
0
transitIon is sensed between 7.4 rnA
an~
g.4 mAe There is
always at least 0.82
rnA
of
hyster~sis
in the receiver.
3.9.1.3 Current flowing in the receiver is interpreted as a mark. By grounding
INI (P2, Pin 2), current flowinq in the receiver can be interpreted as
a space.
3.9.'2
The current loop transmitter (Out) consists of a 20-rnA dc current
source and a
20-rn~
dc current sink.
The transmitter circuits are switched on and off to provide a high out-
put impedance 20-mA de current loop transmitter. The CLA line (P2, Pin
6) will source 20 rnA when on. The current path Is from the +t2V supply
out the CLA pIn, and into the receiver. The return path Is ground.
The_eLP line (P2, Pin 7) will sink
~O
mA dc_when on. The
c~rrent
path
Is from the receiver into the CLP pin, and into the -12V supply. The
return path is qround. This transmitter will source current into +7.5V
to -12v
(re~erenced
to ground) or sink current from +t2V to -7.5V (ref-
erenced to qround).