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Examples of Interface Sequences between the IBM 3704 and 3705 and the Commu-
nication Line
Figures 6-10 show examples of timings and sequences which can be used to
communicate with a sampling of IBM terminals, connected through typical
modems to the IBM 3704 and 3705. Timings depend upon the IBM 3704 and
3705 program activity, the type of scanner, the number of lines being serviced, the
type of line set, the line speed, the type of modem, and the remote terminal type.
Loss of Data Set Ready or Clear to Send
Loss of Data Set Ready (in PCF state 5, 7, 8, 9, A, B, C, or D) or loss of Clear to
·send (in PCF state 9, A, B, or D) causes the Modem Error bit (bit 3) to be set in
the ICW. This bit is set in the ICW either at bit service scan time or at character
service queue reset time.
If
the modem error is detected before the Normal Service Request bit is set, the
Modem Error bit will be ON, and the Normal Service Request bit will be OFF at
L2 interrupt time (for character service). If the Normal Service Request bit is ON
when the Modem Error bit is set, the Normal Service Request bit is turned OFF.
If
the line is idle, or a switched line disconnects without a break or space detected,
then a character service interrupt will not be taken. Under these conditions, the
program has to interrogate the ICW periodically to detect a modem error.
The minimum time to detect loss of Data Set Ready or Clear to Send as a modem
error is variable and depends on the line speed. The minimum time to detect a
down level as an error varies from 50 ns to the bit period of the line plus 0.153
ms.
The maximum time to detect loss of Data Set Ready or Clear to Send as a modem
error is as follows:
Line Speed
110.00 bps
134.49 bps
600.00 bps
1200.00 bps
2000.00 bps
2400.00 bps
4800.00 bps
7200.00 bps
Maximum Time
To Detect Loss
9.2ms
7.6ms
1.7 ms
987.0 us
653.0 us
570.0 us
362.0 us
215.0 us
22
IBM 3704 and 3705 Communications Controllers Original Equipment Manufacturers' Information
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