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PCI VELOCITY / OPTO ISO RS422/485
Autogating low level RS485 half duplex (2 wire) control
General PCI RS422/485 Serial Port Features (cont).
Clock Input:
Drivers:
High Level Voltage:
Low Level Voltage:
High Level Current:
Low Level Current:
Receivers:
Difftial I/P threshold: 200mV max.
Hysteresis:
Input Impedance:
Autogating.
Multitasking operating systems, like Windows 95,
Windows NT, OS/2 and UNIX, cannot successfully use the RTS
line to control the gating of the PC's transmitter. Whilst they can
set the RTS line true just before the data transmission starts, they
cannot set the RTS line false quickly enough after the data has
gone. This is due to the time slicing mechanism used by these
multitasking operating systems. The interrupt service routines in
multitasking OS's have a long latency, i.e. time taken to switch
context from the foreground program to the interrupt routine.
During this time, the first few bytes of the external devices reply to
the PC may be lost since the serial port transmitter is still gated
onto the twisted pair cable.
To overcome this problem, Autogating hardware has been
added to the to the RS422/485 card. This automatically detects the
start of the PC's data transmissions, and gates the PC's transmitter
onto the twisted pair cable. It then automatically detects the last
stop bit being sent and gates the port off the twisted pair cable. An
added bonus of RS485 AUTOGATING is that special device
drivers no longer need to be written to use RS485 in any operating
14.7456MHz
SN75174
3.7V typical at 33mA source
1.1V typical at 33mA source
-60mA max.
60mA max.
SN75175.
50mVolt typical.
>12K Ohm without terminators.
Specifications
Chapter 2 Page 7
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