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Battery Backed Static RAM
EPM-4 Reference Manual Reference – 27
Battery Backed Static RAM
The EPM-4 can be ordered with an optional 2 MB of Battery Backed Static RAM (BBSRAM).
This BBSRAM is powered by the boards RTC battery when main power is turned off. Jumper
V1, which is used to clear CMOS RAM, does not affect the BBSRAM.
The BBSRAM is located at the absolute address of (0x08000000) in one continuous 2 MB block.
Serial Ports
The EPM-4 features two on-board 16550 based serial channels located at standard PC I/O
addresses. COM1 and COM2 are RS-232 (115.2K baud) serial ports.
COM3 and COM4 can be operated in RS-422 or RS-485 modes. Two additional non-standard
baud rates are also available (programmable in the normal baud rate registers), up to 460K baud.
Interrupt assignment for each COM port is handled in CMOS Setup, and each port can be
independently enabled or disabled.
All four serial ports are protected against ESD damage. This protection exceeds 15KV human
body model.
COM
PORT CONFIGURATION
There are no configuration jumpers for COM1 and COM2 because it only operates in RS-232
mode.
Jumper V2 is used to configure COM3 and COM4 for RS-422/485 operation. See page 23 for
jum
p
er configuration details.
COM3
AND COM4 RS-485 MODE LINE DRIVER CONTROL
The TxD+/TxD– differential line driver can be turned on and off by manipulating the DTR
handshaking line.
The following code example shows how to turn the line driver for COM3 on and off:
mov dx,03ECh ; Point to COM3 Modem Control register
in al,dx ; Fetch existing value
or al,01h ; Clear bit D0
out dx,al ; Turn DTR on (enables line driver)
in al,dx ; Fetch existing value
and al,0FEh ; Set bit D0
out dx,al ; Turn DTR off (disables line driver)