CTI Products EXB-232 Asynchronous Serial System Extender - Manual (Page 19)

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RS-232 Hardware Handshake Controls
The EXB-Serial module uses the hardware handshake control lines to control the flow of data to
and from the RS-232 communication channel. These control signals are listed below.
Control Signal Direction
RTS
Output
CTS Input
DSR Input
DTR Output
The control signals DCD and RI are not used by the EXB-Serial module.
The RTS and CTS lines control the flow of characters to and from the EXB-Serial module. The
EXB-Serial module asserts the RTS output TRUE when it is ready to receive incoming RS-232
data and asserts the RTS output FALSE to signal the RS-232 communication channel to stop
sending data. The CTS signal must be TRUE for the EXB-Serial module to send RS-232 data.
If the EXB-Serial module is sending characters too fast for the RS-232 communication channel,
the CTS signal can be set FALSE by the RS-232 communication channel to hold off the EXB-
Serial module from sending more characters.
The DSR and DTR lines control the channel link between the EXB-Serial module and the RS-
232 communication channel. The DSR input must be TRUE for the EXB-Serial module to begin
sending and receiving RS-232 data (the ERR LED will blink fast if DSR is FALSE). The EXB-
Serial module always asserts the DTR output TRUE.
If the RS-232 communication channel being used does not support these hardware handshake
control signals, you must strap the DTR signal to the DSR and CTS inputs in order for the EXB-
Serial module to operate.
You can also use the Option B switches to disable DSR and/or CTS sensing. See section 2.2.1
for details.
3.4 Standard Systems and Custom Configured Systems
The EXB units pass traffic to and from the MCN modules in both directions.
Standard Systems are defined as systems having 20 or fewer comparator and I/O modules
(CIBs, AIBs, IOBs). Custom Engineered Systems are systems that have greater than 20 of
these modules or systems that require a 1.25 Mb network.
EXB modules in standard systems are programmed as repeaters to pass all messages in both
directions. EXB modules in Custom Engineered Systems are custom configured to pass certain
traffic in certain directions only. This reduces the bandwidth required over the Telco or RS-232
link. If you have a Custom Engineered system , see your custom system documentation for
details on re-programming the custom-configured EXB modules after repairing or replacing.