56
The IPLI and IPLO state contains these registers:
Name
Size
Radix
Action
IBUF
16
8
Input buffer
OBUF
16
8
Output buffer
CTL
1
2
Control flip-flop
FLG
1
2
Flag flip-flop
FBF
1
2
Flag buffer flip-flop
HOLD
8
8
Holding buffer
TIME
24
10
Polling interval for input
4.10 12920A Asynchronous Multiplexer Interface
The HP 12920A Asynchronous Multiplexer Interface is a 16-channel terminal multiplexer commonly used with the
HP 2000 series of Time-Shared BASIC systems. It connects from 1 to 16 serial terminals or modems at
programmable baud rates from 75 to 2400 bits per second. Character sizes are also programmable from 5 to 12
bits in length, including the start and stop bits. Each channel can be independently configured for separate send
and receive rates. The multiplexer is not buffered, so the CPU has to retrieve each character from a given channel
before the next character arrives. To avoid saturating the CPU with interrupt requests, the multiplexer maintains an
internal "mini-interrupt" system that queues requests and holds additional interrupts off until the CPU acknowledges
the current request.
The 12920A consists of two Printed Circuit Assemblies — a Lower Data PCA and an Upper Data PCA — and an
optional Control PCA. In simulation, the multiplexer is implemented as a three-device set, as follows:
•
MUXL — the Lower Data PCA
•
MUX — the Upper Data PCA
•
MUXC — the Control PCA
The Data PCAs provide the serial data line connections for terminals and data sets. Five additional receive-only
auxiliary channels may be connected as a group under software control to one of the sixteen main channels to
detect the incoming baud rate. The Control PCA provides serial control and status lines for Bell 103 data sets.
The Lower and Upper Data PCAs must be in adjacent I/O slots. The Control PCA may be placed in any slot,
although in practice it was placed in the slot above the Upper Data PCA, so that all three PCAs were physically
together.
The 12920A supported one or two Control PCAs (two were used with 801-type automatic dialers). Under
simulation, only one Control PCA is supported.
4.10.1 Lower and Upper Data PCAs
The Data PCAs provide the Transmitted Data (BA) and Received Data (BB) lines for up to sixteen terminals. They
perform input and output through Telnet sessions connected to a user-specified port or through individually
specified host serial ports. The Data PCAs support concurrent Telnet and serial connections. The
ATTACH
command specifies the local port to be used for Telnet connections:
ATTACH MUX <port>
...where
port
is a decimal number between 1 and 65535 that is not being used for other TCP/IP activities. When
the Upper Data PCA is attached and the simulator is running, the multiplexer listens for connections on the
