Campbell Scientific CR510 Basic Datalogger - Manual (Page 74)

SECTION 6. 9-PIN SERIAL INPUT/OUTPUT
6-4
6.5 MODEM/TERMINAL PERIPHERALS
The CR510 considers any device with an
asynchronous serial communications port which
raises the Ring line (and holds it high until the
ME line is raised) to be a modem peripheral.
Modem/terminals include Campbell Scientific
phone modems, and most computers,
terminals, and modems using the SC32A
Optically Isolated RS232 Interface or the SC932
RS232 DCE Interface.
When a modem raises the Ring line, the CR510
responds by raising the ME line. The CR510
must then receive carriage returns until it can
establish baud rate. When the baud rate has
been set, the CR510 sends a carriage return, line
feed, "
∗
".
The ME line is held high until the CR510 receives
an "E" to exit telecommunications. The ME is
also lowered if a character is not received after 40
seconds in the Telecommunications Command
State (2 minutes in the Remote Keyboard State).
Some modems are quite noisy when not on line; it
is possible for valid characters to appear in the
noise pattern. For this reason, the CR510 counts
all the invalid characters it receives from the time
it answers a ring, and terminates communication
(lowers the ME line and returns to the
∗
0 Mode)
after receiving 150 invalid characters.
6.6 SYNCHRONOUS DEVICE
COMMUNICATION
The CR510 has the ability to address
Synchronous Devices (SDs). SDs differ from
enabled peripherals (Section 6.2.1) in that they
are not enabled solely by a hardware line. An
SD is enabled by an address synchronously
clocked from the CR510. Up to 16 SDs may be
addressed by the CR510, requiring only three
pins of the 9-pin connector.
Synchronous Device Communication (SDC)
discussed here is for those peripherals which
connect to the 9-pin serial port. (This should
not be confused with Synchronous Device for
Measurement (SDM) peripherals. Although the
communication protocol for SDMs is very
similar, their addressing is independent of SDC
addresses and they do not have a ring line.)
SD STATES
The CR510 and the SDs use a combination of
the Ring, Clock Handshake (CLK/HS) and
Synchronous Device Enable (SDE) lines to
establish communication. The CR510 can put
the SDs into one of six states.
STATE 1, the SD Reset State
The CR510 forces the SDs to the reset/request
state by lowering the SDE and CLK/HS lines.
The SD cannot drive the CLK/HS or RXD lines
in State 1, however, it can raise the Ring line if
service is needed. The SD can never pull the
Ring low if a Modem/Terminal is holding it high.
Data on TXD is ignored by the SD.
STATE 2, the SD Addressing State
The CR510 places the SDs in the addressing
state by raising CLK/HS followed by or
simultaneously raising SDE (Figure 6.6-1). TXD
must be low while SDE and CLK/HS are
changing to the high state.
FIGURE 6.6-1. Addressing Sequence for the RF Modem