Radio Shack TRS-80 Service Manual page 63

Thc RS-232C board is a port mapped device and the ports
used are EB to EB. following is a description of each port
on both input and output.
The following list is a pinout description of the DB-25 con·
nector (Pl).
PORT
E8
EA
E9
ES
INI ' UT
Modem status
UART status
Not Usad
Receiver Holding
register
OUTPUT
Master Reset, enables
UART control registor
1000
UART control register
load and modem control
Baud rate reyister load
enable bit
Transmitter Holding
register
PIN#
1
2
3
4
5
6
7
8
20
22
SIGNAL
PGND (Protective Ground)
TD (Transmit Data)
RD (Receive Data)
RTS (Request To Send)
ers
'(Clear To Send)
DSR (Data Set Ready)
SGND (Signal Ground)
CD (Carrier Detect)
DTR (Data Terminal Ready)
RI (Ring Indicate)
Interrupts are supported on the RS-232C option ooijrd by
the Interrupt mask register (Ul0) and the Status register
lUg) which allows the CPU to see which kind of interrupt
has occurred. Interrupts can be generated on receiver data
register full, transmitter register empty, and anyone of the
errors - parity, framing. or data overun. This allows il min-
urnUIll of CPU overhead in transferring data to or from the
UART. The interrupt
ma~
register is port E0 (write) and the
interrupt status register is port E0 (rcild). Refer to the 10
Port description fOI" a full breakdown of all interrupts and
their bit positions.
The Model III RS·232C board is functionally identical to
the Model I RS-232 bOClrd with the following exceptions:
Interrupts are supported, there are no S€nse switches for con·
figuring the intcrface, there is no COMn-ERM switch for re-
versing the function of pins 2 and 3 on the D8·25, and the
DC to DC converter b not required since +12V and -12V are
provided by the internal power supply. Other differences
include three additional interface outputs and no crystal for
the BRG. All Model I software written forthe RS·232 inter·
face is compataule with the Model II I R5-232C option board,
provided that the software does not use the sense switches to
configure the interface. The programmer can get around this
problem by directly programming the BRG and UART for
the desired configuration or by using the SETCDM command
of the disk operating system to configure the interface. The
TRS·aO RS·232C Interface hardware manual has a good dis-
cussion of the AS·232C standard and specific programming
examples (Catalog Number 26·1145).
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