printer executing each command then stopping

I have configured Marlin 1.1.8 for many printers, of my design and for kits, but this symptom beats anything I have seen before.
I yanked my Anet A8 controller's SD card holder off the board (oops) and replaced it with a Rumba controller and the more standard reprap discount smart controller with a knob. Then I pulled a vanilla Marlin 1.1.8 and configured the relevant settings to mirror the behavior of the Marlin A8 firmware example.
But
I must have goofed something.
When I send the gcode, via USB from Simplify3D to the printer, it will heat the bed, then heat the hot end, then sit there saying "heating E1". If I tell S3D, via the control panel, to force the next step, then the printer will do the next step, home the head. Then sit there until I again "force the next step", then it will do that... Rinse, repeat.
If I run gcode from the SD card, things progress normally. So, this appears to be a serial port issue.
It kind of feels like the firmware is expecting a handshake in the serial protocol, but I did not configure that. I will look for that kind of thing in the firmware but as a parallel effort, if anyone out there has experienced this and figured it out - I am all "ears".

be well,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.

Are you sure the USB baud rate is correct? My printer will successfully connect at either 115200 or 250000, but will only accept G code at 250000 baud. Just a thought. Prits fine from SD card.

Quote
rq3
Are you sure the USB baud rate is correct? My printer will successfully connect at either 115200 or 250000, but will only accept G code at 250000 baud. Just a thought. Prits fine from SD card.

I didn't say it wouldn't connect, I said it would stop executing commands. Each command sent returns an "ok" in the communications window, but the slicer never sends the next command to the controller.
S3D is working fine and connects and communicates gets all of the information that the printer sends out, which it sends out whether or not something is listening. And S3D is sending USB print jobs just fine to other printers.
BUT
When printing a file, .
Something I set in configuration.h or configuration_adv.h must have been the wrong thing to do.

be well,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.

Found it. I do not understand why this caused a problem.

// Host Receive Buffer Size
// Without XON/XOFF flow control (see SERIAL_XON_XOFF below) 32 bytes should be enough.
// To use flow control, set this buffer size to at least 1024 bytes.
// :[0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048]
//#define RX_BUFFER_SIZE 16

When I comment out the //#define RX_BUFFER_SIZE line, things come back to life. I did not enable xon/xoff, just defined the receive buffer.

OK, whatever, things are working fine again now, so I'll just document that oddball consequence.

be well,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.