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USING MACHINE OPTIONS
Clear-to-Send Delay
The 3872 clear-to-send delay has options of 8.5ms, 25.0ms,
75.0ms, and 150.0ms, nominally. The option is selected by
strapping, as follows:
8.5ms - Used on multipoint tributary modems, on non-
switched (leased) channels, when 'new sync' is
used at the control station or when the control
station DTE waits for the fall of carrier before
polling another tributary. IBM 2845/2848
tributaries use 8.5ms.
25.0ms - Used on multipoint tributary modems, on non-
switched (leased) channels, when 'new sync' is
not used at the control station and polling is
immediate.
75.0ms (110.0ms with Switched Network feature)- Pro-
vided for two-wire, half-duplex short lines in
World Trade countries and, in individual cases
on two-wire, half-duplex non-switched or
switched network channels in the USA or
Canada, where turnaround time is critical and
communications facilities characteristics are
suitable.
150.0ms (180.0ms with Switched Network feature) - Used
on two-wire, half-duplex non-switched channels
and for USA and Canadian switched network
operation.
Note 1: On a 3872 with the Switched Network feature,
or on a 3872 with the Switched Network Backup feature
wired to operate with a remote 3872 having the Switched
Network feature, the clear-to-send delay for the first
transmission of each call in each direction is 1020ms.
Note 2: For clear-to-send delays of 25.0ms or longer,
carrier is not turned on until 14.Sms after 'request to
send' rises (is turned on).
Data Carrier Detected and Echo Clamp Delays
Echo clamp is provided to prevent the reception of reflected
data during turnaround, for operation on two-wire, non-
switched channels or on the switched network. The echo
clamp at the formerly transmitting modem is shorter than
the clear-to-send delay at the newly transmitting modem.
Note: The switching of the 'data carier detected' inter-
face line from down (off) to up (on) does not occur at
the expiration of the echo clamp delay. Instead, it occurs
at the time of the first space-to-mark transition after the
delay expires.
Loss of Carrier (CD Bridge) Options
The response (to loss of carrier) of the 'data carrier detected'
DTE interface line from a receiving 3872 is optional and is
mainly a data system consideration. Option 1 provides
maximum data recovery when loss of carrier is less than
8.5ms. Option 2 provides maximum data recovery for
systems that may abort the message if 'data carrier detected'
falls; this option maintains 'data carrier detected' (but
clamps 'received data') when carrier loss is less than 8.5ms.
Option 3 assumes that any loss of carrier will abort the
message; this option is recommended and is factory-strapped,
as it provides maximum accuracy and control of data transfer.
The options, selected by strapping, are as follows:
1. 'Data carrier detected' turns off (drops) 1.8 - 2.3ms after
loss of carrier. If carrier resumes within 8.3ms, 'data
carrier detected' turns on (rises) within 1.8 - 2.3ms.
Otherwise, the normal delay occurs.
2. 'Data carrier detected' turns off (drops) more than 8.5ms
after loss of carrier. If carrier resumes, the normal turn-on
(rise) delay occurs. 'Received data' clamps to a mark level
within 1.8 - 2.3ms after loss of carrier and is undamped
within 1.8 - 2.3ms, if carrier resumes within 8.5ms.
3. 'Data carrier detected' turns off (drops) 1.8 - 2.3ms after
loss of carrier. If carrier resumes, the normal turn-on
(rise) delay occurs. For 1200 bps operation, this option
is automatically selected by the 3872, and the interval
is 2.9 - 3.8ms.
Modem Environment
3-7
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