Autogating - Brainboxes CC-357 User Manual

PCI Velocity RS232422/485
PCI RS422/485 Serial Port Features
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 system. To all intents
and purposes, it looks just like an ordinary RS232 COM port to the
host PC. Autogating for half duplex operation is a necessity for
Windows 95/98, NT and the forthcoming Windows Millennium and
Windows 2000.
14.7456MHz
Æ
SN75174
3.7V typical at 33mA source
1.1V typical at 33mA source
-60mA max.
60mA max.
SN75175.
50mVolt typical.
>12K Ohm without terminators.
Chapter 2 Page 12
HARDWARE SETUP