How to configure an IBM 7855 modem for RTS/CTS ?
IBM 7855 modem attached to 8-port adapter
DESC: They have hooked up a 7855 modem to a machine that was 3.2.5. While
it was 3.2.5 the modem worked just fine. They then upgraded the machine
to AIX 4.1.4 with the same modem and now, 85% of the time the modem
works great. However, the other 15% of the time, it will just lock up
and it will appear as if connection has been lost. But when she
unlocks, all of the info show on her screeen like its been buffered,
and the commands execute correctly. Sometimes the machine will just sit
in what appears to be a locked state for 2 or 3 minutes, then just free
Since they upgrade the system to AIX 4.1.4, they are now
experiencing a slow performance with the modem. It would wait for
a few minute before executing the command. When it do execute like
a list command it would display one letter at a time slowly. When
you try to change a word within a file the cursor would move to the
beginning of the line than to the end of the line. The tty is set
at 19200 bps for a dial in only.
They did remove and re-added the tty and that did not make
any different. She notice that when a 2400 baud rate modem
dial in has no problem, but when she and the system analysis
dial in from home on a PC with a modem of 28.8. Experience
Check that the modem parity, bit and stop bit were set the
same on both of the modem. (8, 1, none). Try to check the
baud rate setting on the 7855, the DTE speed is set to 19200.
After upgrading to AIX 4.1.4, when you dial into the 7855 modem
you intermittently experience a few seconds of delay before your
keystrokes are echoed back to you... This happens once every 10-15
The 7855 is attached to an 8-port async adapter (MCA)
Changed a few things in your tty configuration:
. was changed to
flow control none rts
TBC 16 64
receive trigger 3 2
lslpp -h bos.rte.tty shows 220.127.116.11 level. Sent IX60462 (U445541) to get
the 18.104.22.168 level to you.
Suggested the above changes to the tty configuration. Also, when the
problem occurs, check the "DQ" (data quality) indicator on the 7855
modem to see if it is experiencing retransmits due to line noise.
Customer altered read trigger and flow control and saw no improvement.
Applying the PTFs didn't do the trick.
I dialed in. Your tty (tty1) is at 19200, using RTS/CTS
I run lptest command and I see intermittently some lines being
dropped. The DQ (data quality) indicator on the 7855 never
changes from 0 (good line). Checked flow control setup on the
7855. It was set to RFS to stop DCE (RTS) but none for the flow
control to be used to stop DTE. Set the flow control to stop the
DTE to RFS (RTS) and tried again... Still having data loss.
Asked if you could swap out modems to see if we can narrow this down
to whether or not this is the configuration on the 7855 causing the
problems. You will track down another modem and call back.
Connected US Robotics Sportster modem and configured it properly.
We tried dialing into your system when you had the US Robotics
modem attached and had no data loss or tty overrun errors being logged
so we know its the modem configuration, not an AIX problem.
We put the 7855 modem into asynchronous AT mode with the front panel
arrows as follows:
To set up the modem to accept AT commands, simultaneously
press the right and left arrow keys on the front of the
modem. The front panel should then read:
Press the right arrow to Enter, and then press the down
arrow until you see "First Setup". Then press the right
arrow once. Press the up arrow until you see "Asynchronous
AT", then keep pressing the left arrow to exit out of the
Then, using the command "cu -ml tty\#\#"
send the modem AT\\Q2\\R2&W to put it into RTS/CTS flow
control (RFS/CFS according to the modem manual).
After the above configuration changes to the IBM 7855 modem,
customer was able to successfully dial in and experienced no garbage or
data loss after sending the above two AT commands to the 7855 modem.
Support Line: How to configure an IBM 7855 modem for RTS/CTS ? ITEM: CG2029L
Dated: November 1996 Category: N/A
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