Manual 104i-COM-8SM
12
Most PCI bus signals are common to all four cards in the PCI stack. However, there are four
unique signal groups, one for each card. The slide switches select which signal group goes
to each card. The card in the stack closest to the CPU board must get signal group 0.
Only four PCI boards are allowed in a PC/104-Plus stack, each board must get a specific
set of signals. These signals are selected with two slide switches, labeled SEL-1 and SEL-
2, which form a binary value to control the mux (SEL-1 is the least significant bit and SEL-2
is the most significant bit). If this card is furthest from the CPU, slide both switches to the
OFF STATE. This will select the signal with the longest trace on the CPU board (signal
group 3). If this card is closest to the CPU, slide both switches to the ON STATE. This will
select the signal with the shortest trace on the CPU board (signal group 0). Place the SEL-
1 switch to ON and SEL-2 to OFF to select signal group 2, place the SEL-1 switch to OFF
and SEL-2 to OFF to select signal group 1.
Jumper Function
(refer to option selection map)
Each communication channel (COM A through COM H) has four configuration jumpers.
Four modes are possible: RS-232, RS-422 (also 4 wire RS-485 Master), 2 wire RS-485,
and 4 wire RS-485 Slave. The communication modes are implemented with an RS-232
transceiver, a differential signal half-duplex transceiver (auto-RS-485 mode Tx/Rx and RS-
422 Tx), and a differential signal receiver (RS-422 Rx).
The first jumper enables the RS-232 transceiver.
The second jumper connects the auto-RTS signal from the UART to the transmit-enable pin
on the half-duplex differential transceiver or it connects either a permanent ON (for RS-422)
or a permanent OFF signal (for RS-232) to the transmit-enable pin.
The third jumper enables the RS-422 receiver and sends a permanent ON signal (for RS-
422 Tx) to the transmit-enable pin on the half-duplex differential transceiver.
The fourth jumper either disables the half-duplex differential transceiver's receiver (not used
for RS-422, 4 wire RS-485, or RS-232) or connects the auto-RTS signal to the receiver's
enable pin (2 wire RS-485).
RS-485 Balanced Mode Operation
The board supports RS-485 modes that use differential bus transceivers for increased range
and noise immunity. The RS-485 specification defines a maximum of 32 devices on a single
line. The number of devices served on a single line can be expanded by use of “repeaters”.
The board also has the capability to add load resistors to terminate the communications
lines. RS-485 communications requires that one transceiver supply a bias voltage to ensure
a known state
(an “idle one”) when all transmitters are off. Also, receiver inputs at each end
of the network
should be terminated to eliminate “ringing”. The board supports biasing by
default and supports termination by jumpers on the board. If your application requires the
transmitter to be un-biased, please contact the factory.
The driver/receiver used, type 75176B, is capable of driving extremely long communication
lines at high baud rates. It can drive up to ±60 mA on balanced lines and receive inputs as
low as 200 mV differential signal superimposed on common mode noise of +12 V or -7 V. In
case of communication conflict, the driver/receivers feature thermal shutdown.
