ND Holmes : MRBus MRB-SCSN CTC Siding Control Node (CSCN) browse

CTC Siding Control Node (CSCN) Introduction

Centralized Traffic Control (CTC) is a great many things, but simple definitely isn't one of them. For those unfamiliar with CTC signaling systems, I'd suggest reading a bit about it first, as it's a rather complex subject. Some webpages that might help are Logic Rail Technologies' CTC Signaling Page, Carsten Lundsten's CTC basics, and Rich Weyand's Advanced CTC Signaling page. In addition, there have been a number of printed articles and books over the years, but since those aren't online they're not much use to you right now.

The MRB-CSCN (CTC Siding Control Node) encapsulates all the signaling logic and drivers for two CTC control points arranged to form a siding. While it might seem more logical to wire each CTC control point individually and then interlock them with rules and traffic over the bus, from a model and prototype point of view, there's usually very little point to having a controlled CTC point at one end of a siding and then leaving the opposing end out of the system. Since fourteen of the fifteen control points I have to wire on the test layout are nice, normal sidings, I decided to design a board to manage all the necessary elements of signalling a siding. Included are drivers for twin-coil machines on both turnouts, outputs for four searchlight-style heads with three-leaded bi-color LEDs (this will be modified in the future to expand to other signal types), sense lines for the two turnouts, and inputs for "manual" (switches local to the site) turnout control.

For those of you with slow-motion type motors or three-light signal heads, you'll just have to be patient. Rest assured I'm working on such things, as my own layout (if I ever get to it) will include both of those. I will be growing the MRB-CSCN to include control for these and more in time.

CTC Siding Control Node (CSCN) Introduction

Block Detection Circuits

The block detection circuits are the exact same circuits as are used in the MRB-BD4 detectors. Please see the MRB-BD4 pages for details on their operations, credit for their design, etc.

Capacitive Discharge Circuit

Now, as for the capacitive discharge switch machine driver, it was actually developed as a standalone circuit. Take a look at this schematic for just the CD circuit. As you'd expect, C1 stores charge, and Qx (the Teccor TC22-4 SCRs) release it through the coil upon receiving a trigger pulse to the gate. Q4 controls the charging of the input capacitor. Simple enough, right?

D1 and D2 detect when current is flowing through the coil. Any current will produce a ~1.2V potential at the common point between R6 and R7. R7 feeds current to Q1, which drives Q2 and Q5. These turn off Q4 (the charging transistor) as long as current is flowing through one of the coils.

The circuit would work with just that much, but there's a problem: It takes far too long to drain the capacitor of all charge (or enough to stop current flow and reset the SCRs, anyway). Most of the useful energy is expended in the first few milliseconds, as this is when the current flow is greatest and thus the magnetic field is strong enough to do some productive mechanical work. Because it's an RC circuit, essentially, most of the stored energy is used within the first 1/3 of the cycle. Once C1's voltage drops below 5.6V (about a 1/3 of supply voltage), Q6 turns off and allows Q7 to dump capacitor energy until current shuts off. This significantly improves cycle time. The circuit is now fast-cycling, yet perfectly safe - it won't recharge until the trigger pulse is removed and current stops flowing. It works quite nicely by itself, if you don't want to build the whole CTC node.

Documentation and Source Code

Documentation

CSCN Configuration Values and Packet Summary
A guide to the configuration values on the CSCN and the structure of the network packets it sends and accepts.

[pdf]

Schematics

CSCN Schematic Version 1.0 Final MRB-CSCN schematic	[png]
Capacitive Discharge Circuit Part of the CSCN, perfect for standalone use	[png]

Firmware

Version 1.2 Finished MRB-BD4 firmware, incorporating the bus-lockup fix	[src] - [hex]
Version 0.99 (Obsolete - Reference Only) Initial Release	[src]

Photos

	Hand-wired Prototype Installed at Carson on Wind River RR	[960x640] [2160x1440]
	Workbench with Prototype My makeshift test rig on my workbench	[960x441] [2160x991]