PCI Quad 422/485 Card
Chapter 2 Hardware Configration
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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, The 16950 has on chip autogating. This option
requires a special device driver (supplied) 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 but to an application on the host PC. It looks just like
an ordinary RS232 COM port. Autogating for half duplex operation is a
necessity for Windows 95, NT and Windows 2000.
Card Layout and Specifications.
PCI Quad Opto RS422/485 Specifications:
Dimensions: 4.2 x 7.8 in, 106 x 205 mm
I/O Connection: 37 way female D connector to 4 serial ports
Serial Port 1: 9 pin Male D type.
Serial Port 2: 9 pin Male D type.
Serial Port 3: 9 pin Male D type.
Serial Port 4: 9 pin Male D type.
