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Key Super Serial Card Memory Locations for Programmers
Note: To correctly view tables and diagrams on a super-res display,
use a mono-spaced Font such as CoPilot or PCMononspaced.
____________________________
Key Super Serial Card (SSC) Memory Locations for Programmers
by Aaron Heiss
All addresses are of the form '$C0sx', where 's' is the Slot number
of the SSC plus $8 and 'x' is the final digit ($0 - $F) of the
address.
Control Register
The 6551 Control Register is mapped to $C0sB ($C09B for Slot 1, $C0AB
for Slot 2, etc.). Its format is ...
bit 7 : stop bit number [0= 1 stop bit, 1= 2 stop bits for most
settings, except: 1.5 stop bits for word length 5 and no parity,
and 1 for word length 8 and parity]
bits 6 and 5 : word length [00= 8 bits, 01= 7 bits, 10= 6 bits,
11= 5 bits]
bit 4 : receiver clock source [0= external, 1= internal]
bits 3, 2, 1, 0 : baud rate, as follows:
0000= 16x external clock
0001= 50 bps
0010= 75 bps
0011= 109.92 bps
0100= 134.58 bps
0101= 150 bps
0110= 300 bps
0111= 600 bps
1000= 1200 bps
1001= 1800 bps
1010= 2400 bps
1011= 3600 bps
1100= 4800 bps
1101= 7200 bps
1110= 9600 bps
1111= 19200 bps
Note that 1 MHz Apples (everything other than the Apple IIgs and //c
Plus running in "fast" mode) cannot handle 19.2 kbps, and even 9600
bps on these machines requires either some highly optimised code or
a decent buffer in the device being accessed. The faster Apples
have no difficulty with this speed, however.
Command Register
The 6551 Command Register is mapped to $C0sA, and is essentially an
extension of the Control Register. Its format is ...
bits 7 and 6 : parity mode control [00= odd parity, 01= even
parity, 10= mark parity (parity bit always set), 11= space parity
(parity bit always cleared)]
bit 5 : parity mode enable [0=no parity used, 1=parity used]
bit 4 : receiver mode echo [0=no echo, 1=echo]
bits 3 and 2 : transmitter interrupt control [00= set RTS high and
transmit no interrupts, 01= set RTS low and transmit interrupts,
10= set RTS low and transmit no interrupts, 11= set RTS low and
transmit break signals instead of interrupts]
bit 1 : interrupt request disable [0=enable receiver interrupts,
1=disable receiver interrupts]
bit 0 : Data Terminal Ready (DTR) setting [0=set DTR high (indicates
'not ready'), 1=set DTR low (indicates 'ready')]
Note that, although the DTR is generally not used in the SSC (it may
actually not be connected!), it must be set to 'low' in order for the
6551 to function correctly. Also, the RTS signal must be set 'low'
in order to receive any incoming data from the serial device.
Note also that both the Command and Control registers are write-only,
which means that reading them will not necessarily yield valid data
on the device's settings. If there is any doubt as to the current
settings of the 6551, you should (re)initialise them.
Status register
The 6551 Status register is mapped to $C0s9. It is a read-only register
and its format is ...
bit 7 : interrupt [0=no interrupt, 1=interrupt]
bit 6 : Data Set Ready (DSR) [0=DSR low (ready), 1=DSR high (not ready)]
bit 5 : Data Carrier Detect (DCD) [0=DCD low (detected), 1=DCD high
(not detected)]
bit 4 : transmitter data register empty [0=not empty, 1=empty]
bit 3 : receiver data register full [0=not full, 1=full]
bit 2 : overrun error [0=no error, 1=error]
bit 1 : framing error [0=no error, 1=error]
bit 0 : parity error [0=no error, 1=error]
Data Register
The Data Register at $C0s8 is used both for outgoing and incoming data.
Whether the data has been sent or received can be determined by checking
bits 4 and 3 (respectively) of the Status register.
I hope that this is the SSC programming information that you are
looking for. Good luck with your project!
Aaron Heiss
heiss-AT-pdx-DOT-edu
http://web.pdx.edu/~heiss/