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TS-2200 User's Manual
I/O 75h, bit 0 can be read to determine whether the SRAM option is installed; a '1' in bit 0 indicates
that it is installed, a '0' that it is not.
5 Serial Ports
The two PC compatible asynchronous serial ports provide a means to communicate with external
serial devices such as printers, modems, etc. Each is independently configured as a standard PC
COM port which is compatible with the National Semiconductor NS16C450. COM1 appears in the I/O
space at 3F8h – 3FFh and uses IRQ4. COM2 is located at 2F8h – 2FFh and uses IRQ3.
The COM ports use a master clock of 1.8519 MHz as compared to a standard clock of 1.8432 MHz.
This results in an error for all baud rates of .0047 (less than ½ %). The error is insignificant and this
clock value allows standard baud rate selections -- for example a divisor of 12 yields 9600 baud.
By changing an internal configuration register in the 386EX, the serial clock can be switched to 12.5
MHz (the processor clock divided by 2). This feature allows baud rates higher than 115 Kbaud (up to
781 Kbaud), as well as low error, non-standard lower baud rates (such as 24 Kbaud). See Appendix F
for further information.
The COM ports may also be configured to use a DMA channel, which is handy when very high baud
rates are being used. When enabled, a DMA request is issued any time a serial port's receive buffer is
full or its transmit buffer is empty. This allows higher speed operation with much lower CPU overhead.
See the Intel 386EX User's Manual for further details.
5.1 Serial Port Configuration Registers
Because both serial ports are 100% PC compatible, software written for the PC that accesses serial
ports directly or through standard BIOS calls will work without modification on the TS-2200. The
details of the COM port internal registers are available in most PC documentation books or the data
sheet for the National Semiconductor NS16C450 may be consulted.
5.2 Serial Port Hardware
Each serial port has 4 lines buffered: the two data lines
and the CTS / RTS handshake pair. This is quite suffi-
cient to interface with the vast majority of serial devices.
The serial lines are routed to 10 pin headers labeled
COM1 and COM2. A serial adapter cable can be
plugged into the header to convert this into a standard
DB9 male connector. The pin out for the 10 pin header
and DB9 male connector are listed below. The RTS
signal also drives the DTR pin on the serial ports; DTR
is always the same state as RTS. In addition, RTS is
also used to enable the RS-485 transmitter (see below
for more details).
5.3 RS-485 / RS-422 Support
An option is available to add support to COM1 for half or full-duplex RS-485. RS-485 drivers allow
communications between multiple nodes up to 4000 feet (1200 meters) via twisted pair cable. Half-
duplex RS-485 requires one twisted pair plus a Ground connection, while full-duplex RS-485 (also
known as RS-422) requires two twisted pairs plus a Ground connection.
For half-duplex operation, a single twisted pair is used for transmitting and receiving. The serial port's
RTS signal controls the RS-485 transmitter/receiver (see next section for automatic operation). When
RTS is asserted true (bit 1 of the modem control register = 1), the RS-485 transmitter is enabled and
the receiver disabled. When RTS is de-asserted the transmitter is tri-stated (disabled) and the receiver
is enabled. Since the transmitter and receiver are never both enabled, the serial port UART does not
receive the data transmitted. The transmitter and receiver share a single pair of signals that are
NC 10
NC
[in]
CTS
[out]
RTS
NC
Figure 1 - Serial Port Header and DB9 Pin-
out [signal direction is in brackets]
PLEASE NOTE: The serial port headers
use a non-standard numbering scheme.
This was done so the header pins would
have the same numbering as the corre-
sponding DB-9 pin; i.e. pin 8 (CTS) on the
header connects to pin 8 on the DB-9
7
Technologic Systems
5
GND
9
4
DTR (RTS) [out]
8
3
TX data
[out]
7
2
RX data
[in]
6
1
NC
11/06/2000
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