IBM Series 1 User Manual page 130

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There are two different current loop interface "ports"
available on the teletypewriter adapter card. The isolated
current loop interface requires the user to supply power to
drive the two isolated current loops.
A
separate power
supply and current limiting resistor must be placed in the
transmit loop and the receive loop. Refer to
"Teletypewriter Device Information" in a subsequent section
of this chapter for connection of these power supplies and
resistors.
The
teletypewriter adapter supplies the power to drive
the transmit and receive current loops when the non-isolated
current loop interface is used. There are
two
other
significant differences between the isolated and
non-isolated current loop:
1)
On the isolated current loop, the receive inputs are
totally isolated.
On the non-isolated current loop, the receive inputs
are coupled through resistors to
+12
volts and
-12
volts generated
by the card file pover supply.
For both isolated and non-isolated current loops,
transmit minus is logic ground.
2)
On
the isolated current loop, the transmit output is a
£Yllen! sink.
On the non-isolated current loop, the transmit output
is a current
§QY~.
See Figures 4-5 and 4-6 in the "Electrical Characteristics"
section of this chapter for more details of the four
different interfaces available on the teletypewriter adapter
card.
Summar~. Generally, unless the user has some unique
requirements, he should use the non-isolated current loop
interface.
The following table outlines the advantages and
disadvantages of each of the four interfaces incorporated in
the teletypewriter adapter card:
Teletypewriter Adapter 4-7