closed loop speed mode

I have an AX2550 with an encoder module that I installed, connected to two motors, both of which have optical shaft encoders. I am trying to run the controller in closed loop speed mode. However, when I set that mode (by sending RS232 commands, not using Roborun), motor A stands still while motor B accelerates to full speed until I cut the power or set the controller back to open loop mode. I tried switching the channel A and B pins from motor B's encoder, but it did the same thing.
I also tried unplugging motor B. When I did that and set closed loop speed mode, motor A started to twitch, going forward slowly in spurts. I kept querying the speed with ?z and it moved between 0 and 1 (the Time Base is 10). When I sent the command !a10, the motor jumped to full speed for a few seconds, then suddenly cut out and began twitching again (I suspect overvoltage caused it to stop).

The command I am using to set closed loop speed mode is ^01 23, followed by ^FF.

What is going on?

This sounds like a case of encoders be wrongly connected and/or reversed motor polarity.

Use the Roborun utility to troubleshoot. Configure the controller in open loop then click on the encoder tab, click on Run. Verify that you see the correct channel counting up and down as you spin the motor (manually or with the slider).

Then move the slider to spin the motor. Verify that the sign of the speed matches the one from the command. If reversed, you can either swap the encoder signals or invert the motor polarity.

Set the controller to closed loop (with encoder feedback) after that.

Thank you, got it working. I switched some more encoder leads and switched the bit order of the command (again) so it is
^01 C4
to set closed loop speed mode.