Furuno FA-100 Operator's Manual page 58

APPENDIX
Serial interface I/O circuit
1. Input/Output Port SENSOR1, 2, 3, L/R, PC IO (PI), EXTRA1
(Transmission rate is selectable between 4800 bps and 38.4 Kbps.)
Input（ （ （ （ RD - A, B） ） ） ）
Output (TD - A, B)
Logical High: A-B>+4.0V at i
i is normally limited less than 100 mA on this device.
o
This means load resister equal to more than 50 ohms.
2. Input/Output Port BEACON, EXTRA (maker's option)
These two ports are simply used Opt-coupler to isolate I/F only on RD terminal.
TD is not isolated and needed isolation by receiver's responsibility.
AP-8
lsolated IO device
LTC1535
Logical High: A-B>+0.2V
240 ohms can be neglected by jumper plug.
lsolated IO device
LTC1535
RS422A device
SN75C1168
Input current is about 17 mA in case of A-B=+5V.
logical high: A-B< about +2V
RS422A device
SN75C1168
logical High: A-B>+0.2V
i is normally limited less than 100 mA.
o
This means that load resister must be more than 50 ohms.
240 ohm
470 ohm x2
logical Low: A-B<-0.2V
i
o
TD-A
TD-B
470ohm x2
GND isolated
=100 mA logical Low: A-B>-4.0V at i
o
220 ohm
logical Low: A-B> about +2V
i
o
470 ohm x2
logical Low: A-B<-0.2V
RD-A
RD-B
GND isolated
=100 mA
o
RD-A
RD-B
TD-A
TD-B
GND not isolated