IBM 1 Series Manual page 128

TTL. One connector pin is available for TTL-level input. The TTL input is
nonisolated.
ETA. One pin is available for ELA RS232-C level input. This input is
nonisolated.
Grounding. The cable shielded-ground wire must be connected to frame
ground at the point of entry into the IBM enclosure and at the user's
device end of the cable.
Because the performance of this interface may be affected by noise,
appropriate arc suppression, noise filtering, etc., may be necessary on
customerinputs.
Contact sense inputs are isolated from card ground. TTL and EIAsignal
groundsaretied to logic ground in the teletypewriter adaptercard.
Output Circuits General Description
Cable Length
Figure 4-3 shows the four basic types of outputs available: current driver
for data, solid-state switch/TTL for data, solid-state switch/TTL for
control, and EIA RS232-C voltage level. The outputs are selected by
utilizing the appropriate top-card connector pins. All outputs are driven in
parallel; jumpering on the card is not necessary for selecting an output.
Solid-state switches are provided with a resistor and diode in series tied to
+5 volts so that they may be used assolid-state switches or as TTL
outputs. Any user voltage greater than +5 volts that is applied to the
Switch output isolates the +5 volts from the switch by reverse biasing the
series diode.
A 6.1-m (20-ft) cable is provided by IBM to connect the teletypewriter
adapter to a teletypewriter device or a functionally equivalent serial I/O
device. For devices attached to the teletypewriter adapter by either the
EIA-voltage-level interface or the current loop interface, distances
significantly greater than 6.1 m (20 ft) can be achieved, but the maximum
distance achievable depends on cable type, electrical environment, and
device data rate.
Teletypewriter Adapter Feature 4-23