3Com (U.s. Robotics) Modem Bug - Polycom Vortex EF2241 Application Note

3C (U.S. R
OM
In the process of testing various modems to use with the Vortex, we have
discovered a bug that only occurs with 3Com modems. The bug is quite simple:
During an Autoscan, the Vortex will send many commands back to the device
that requests that information and one piece of that information causes the
3Com modem to change its echo mode from off to on. This results in all
information that the Vortex generates on the Transmit pin of the RS-232 port to
be placed on the Receive pin of its RS-232 port. This causes the Vortex to enter
into an infinite loop since all acknowledgments are now processed by the Vortex
as commands.
If you have multiple units connected together (for example 3 EF2280's), and try
to Autoscan the units from a PC, one of the commands that the EF2280 will send
back to the PC will cause the modem to revert into echo mode (E1). For
example, during the the Autoscan, the EF2280 sends this command:
F00PRESETL16,MGATE1,*,000000001100000000
This command causes the 3Com modem to change from no echo of commands
(E0) into echo (E1). Here's a text capture of this event from a terminal window.
Note that the 3Com modem responded with an error message, but the next
characters typed into the terminal window were echoed!
ATI4
U.S. Robotics 56K FAX EXT Settings...
B0 E0 F1
BAUD=9600
DIAL=TONE
&A3 &B1
&M4 &N0 &P0
S00=002 S01=000 S02=043 S03=013 S04=010 S05=008 S06=004
S07=060 S08=002 S09=006 S10=014 S11=070 S12=050 S13=000
S15=000 S16=000 S18=000 S19=000 S21=010 S22=017 S23=019
S25=005 S27=000 S28=008 S29=020 S30=000 S31=128 S32=002
S33=000 S34=000 S35=000 S36=014 S38=000 S39=000 S40=001
OBOTICS
M1 Q0 V1
PARITY=N WORDLEN=8
ON HOOK CID=0
&C1 &D0 &G0
&R2 &S0
) M
ODEM
X4 Y0
&H1 &I2 &K3
&T5 &U0 &Y1
B
UG
14