PCM-2 Manual.pdf - Voss Associates

Al6 is buffered to and from RS-232 levels on connector P4 by A28. This is
currently a spare I/O port. Al7 is used for communications with a host computer
via P3, and may be configured for either RS-232 or RS-485 levels by installing
An or A29, respectively. Only one of these two buffers may be present at a
time.
Parallel Ports
Eight switch inputs are provided, each pulled up to +5 volts. Seven additional
inputs (with pull-ups, but without protection) are distributed across two parallel
I/O connectors and the badge reader interface. Twenty channels of parallel output
are available on three COlmectors. Each is capable of sinking 0.5 Ampere to
approximately I Volt, however some limits exist as to how much total power the
outputs can drive at anyone time.
Parallel inputs are buffered onto the processor bus by 74HC373s at locations A23,
A24 and A25. All off-board inputs are provided with pullup resistors, however
only those on P8 and P9 have resistors and capacitors which offer some protection
against voltage transients.
Parallel outputs are latched into 74HC374s at locations Al9 - A22. Nine
logic-level outpurs are connected directly to P12; four of these are used to send
data and clock signals to the two countdown clock boards. All other off-board
outputs are buffered through ULN2003A chips at locations A32, A33 and A34.
These are open-collector output devices capable of sinking up to 250 mA and
withstanding up to 50 volts. Because these are darlington configuration outputs,
an "ON" state output of 1.0 to 1.5 volts is normal under heavy loads.
Two 4x5 matrix keypads may be connected to PlO and Pll (Pll is not used in
the PCM-2). Only one row of keys is pulled low at a time by the outputs of All
(a 74HC138 one-of-eight decoder), while the five column inputs are scanned as
parallel inputs to A24.
Voltage Regulators
Other Features
Three voltage regulators are used: All, A13 and A31. All provides +5 volts to
most of the front panel board, while A31 powers the RS-232 and RS-485 buffers
and the badge reader if required. The output of A13 is used by the two clock
display boards. These are low voltage drop regulators, and will operate from
inputs of 6 to 12 volts DC applied to Pl.
Interrupts may be generated by the three communications chips under software
control. All three are combined into a single interrupt input at pin 13 of the
microprocessor, and may also be read as parallel inputs to determine which
chip(s) are requesting service.
PCM2.MANI REV AI ~'\;;ril 1995 11-3