&T19 - Perform RTS/CTS Cable Test

Command Line Format
AT [cmds] &T19 <CR>

Suffix Values
N/A

Default
N/A

Stored by &Wn
N/A

Displayed by &V
N/A

Description
DTE software uses this command to determine if the DTE cable for the modem supports RS-232C signals RTS and CTS. Try this test before RTS/CTS flow control (&K3) is used.

The modem does the following when this command is issued:

1. Turns OFF CTS (normally ON) and start a 500-ms timer.

2. Monitors RTS for ON and OFF states.

3. After the 500 ms timeout or when both level-high and level-low states of RTS are detected, restores CTS to the ON state and end the test.

4. If both states of RTS were NOT detected then return the ERROR result code; otherwise issues OK result code.

5. If the timer expires before both states of RTS are detected then:

a. Restores CTS to the ON state.

b. Returns ERROR result code (meaning the modem did not verify RTS).

Note: By itself, the modem cannot verify either signal. The DTE must toggle RTS during the test for the modem to detect a change. Also, the DTE should look for CTS to drop during the test to verify it is connected.

The following steps describe the DTE &T19 test:

Step 1: Send AT &T19 <CR>.

Step 2: Wait for CTS to go off. If it goes off within 500 ms, then go to Step 3. Otherwise the test has failed.

Step 3: Turn off RTS, wait 50 ms, turn on RTS. Repeat up to 6 times.

Step 4: If CTS goes on before the 6 iterations are completed, then go to Step 5. Otherwise, the test has failed.

Step 5: Wait for a result code from the modem. If the result code is OK, then the test has passed. If ERROR result code is received, the test has failed (the DTE-DCE cable is not suitable for RTS/CTS flowcontrol).

You may modify the software algorithm to detect which signal is missing from the cable. After issuing the AT&T19 <CR>command, give the modem about 100 ms to drop CTS. Then, even if the CTS drop is not detected, toggle RTS anyway, remember that the CTS part of the test has failed.

RTS/CTS flow control is preferred because it is &quotout-of-band&quot, that is, it does not use any of the 256 serial codes from the user data flow control uses DC1 (ASCII 17) and DC3 (ASCII 19). If RTS/CTS is not supported, then Transparent Flow Control (&K5) is the next best option (If the software supports it) because it accomplishes flow control without interfering with the user data (e.g., during binary file transfers).

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