Serial Communication

Eight serial ports are provided by the XR17D158 octal PCI UART chip. The input clock is driven by a 14.7456MHz crystal.

Serial Port Operation

Each port can be independently configured for RS-232, RS-422, RS-485 or TTL interface using jumpers or assembly options. RS-232 and TTL are mutually exclusive due to signal level incompatibility. If one transceiver is installed for a port, the other set is not. The standard configuration is RS-232/422/485 transceivers installed. Ports 1 through 4, or 5 through 8, can be configured for TTL line levels as a custom assembly option. Contact Diamond Systems for custom configuration options.

Transceiver control is managed using the eight identical 2x8 jumper blocks, J7-J14.

In RS-485 mode, both local ECHO and NO ECHO modes are supported. In local echo mode the receiver is always on. In no echo mode, the receiver and transmitter are enabled alternately so that no echo is received during transmission. The output transceiver can be automatically enabled/disabled by setting a control bit in the modem status register (MSR) of the XR17D158. This causes RTS to automatically turn on before transmission starts and turn off after transmission ends. Use MSR bits to program the transmitter disable delay after last bit transmission.

The RS-232 and RS-422/485 transceivers have built-in ESD protection. The TTL transceivers do not have ESD protection, however, they serve as protection for the UART in TTL mode.

Line Termination

In RS-422 and RS-485 modes, 150-ohm, 0.25 watt termination resistors may be connected across the differential transceiver lines with jumpers. (One for transmit pair, one for receive pair in RS-422, one only for RS-485).

All UART port inputs have pull-up/pull-down resistors to force the inputs to their inactive state when the RS-422 or RS-485 protocol is selected. All TTL inputs have pull-up/pull-down resistors to force them to their inactive state when unconnected.

All RS-422/485 inputs have jumper-selectable pull-up/pull-down (bias) resistors to force them to their inactive states when unconnected. The pull-up/pull-down resistors must be high enough to prevent a false signal due to the voltage divider resulting from the pull-up, pull-down and line termination resistors (50mV threshold). See the example schematics for line termination and bias resistors, below.

Figure 1: Line Termination Schematics