Digital Inputs; Table 6: Decide Resistance Value; Figure 8: D/I Sample Circuit - CipherLab 5000 Hardware Manual

1.3.9 Digital Inputs

There are totally 4 groups of digital inputs. Each is electrically isolated from the
outside world by a photo-coupler. Current must flow through the internal LED to turn
on the output transistor.
A typical 1 mA turn-on current is recommended.
A sample circuit is depicted below.

Figure 8: D/I sample circuit

How to decide the R value?
Premise: the LED forward voltage is about 1.15 V.
The recommended R is
Kohms when Vext = 5.0 V, and 10 Kohms when Vext = +12 V.

Table 6: Decide resistance value

Introducing 5000/5100:
Vext
external power
R
DIN+
DIN-
external ground
Features
15