Hardware manual

Specifications PCI VELOCITY / OPTO ISO RS422/485
Chapter 2 Page 6
General PCI RS422/485 Serial Port Features (cont).
Clock Input: 14.7456MHz
Drivers: SN75174
High Level Voltage: 3.7V typical at 33mA source
Low Level Voltage: 1.1V typical at 33mA source
High Level Current: -60mA max.
Low Level Current: 60mA max.
Receivers: SN75175.
Difftial I/P threshold: 200mV max.
Hysteresis: 50mVolt typical.
Input Impedance: >12K Ohm without terminators.
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, NT and the forthcoming
Windows 98 (Memphis) and Windows NT 5.