Making a SLC 500 cable

[Corbis]

SLC500 Cable

If I am not mistaken, the 2707-NC1 cable connects form the DTAM's RS485 Port directly to an SLC500 fixed with no protocol conversion within the cable itself.


Regards,
Corbis

 

[Ken Roach]

DH485 is not just RS485, part MCMXLII

Thank you Corbis, for alluding to the basic issue here.

Most RS-232 to RS-485 converters do a fine job of converting the TX and RX pin signals of an RS-232 serial port into transmissions and receptions on the A/B pair of the RS-485 data bus.

Data Highway 485 protocol requires some very precise transmit-enable behaviour of the RS232 port. This is not handled simply by letting the 232/485 converter sense RS485 data and start transmitting on RS232, but rather by performing a Transmit Enable function on the DH485 plug. Take a look at the wiring diagram on a 1747-PIC box.

This is why there are specific RS232/RS485 converters that are used with DH485 protocol. The A-B 1747-PIC is the oldest and most venerable, and the 1761-NET-AIC converter does exactly the same thing with two serial ports, different cabling, and built-in isolation.

Any RS232/RS485 converter that is advertised to replace the 1747-PIC or 1761-NET-AIC includes similar Transmit Enable circuitry.


An additional wrinkle comes from the difficulty of getting the very old 1747-PIC driver to work with Windows NT/2000/XP serial port drivers, and with modern laptop serial ports. That's why A-B offers the USB/DH485 converter, part number 1747-UIC, so all that stuff doesn't need to be messed with.

Last problem: The 1747-UIC works best with DH485 networks, and does not always connnect simply to standalone fixed-IO SLC controllers or 5/01 and 5/02 controllers. This is a double whammy because these ancient controllers are the ones most likely to be in the hands of someone unfamiliar with DH485 networking.

I present my customers with a simple decision matrix.

1. Does your PC have a built-in RS232 serial port ?
If Yes, I will help you set up and use the RSLinx 2.51 "1747-PIC/AIC+" driver. I will not support older versions of RSLinx nor can I attempt to support add-on RS232 ports.

2. Buy a 1747-UIC and I will teach you how to connect it to an old SLC-500 controller by generating extra network traffic that "kickstarts" the DH485 token rotation.

 

[Ken Roach]

Lets leave the poor DTAM Micro out of this and let it be. Its history was short, its volume small.

I'm going to be posting a lot of links to this Knowbase article:

Answer ID # 16672, Explanation : RS232 & RS485 (Standards) and DF1 & DH485 (Protocols)

 

[macrophobia]

I did locate a cable in someones trove of electronic artifacts. I'll spend some time digesting the information gathered in this thread.

When I return from my assignment, I'll upload a photograph of the schematic which is printed on the back of the PIC box. I'm sure it will be of interest, and should solidify the solution for anyone else who has this same issue.

Outstanding support from everyone! I'm glad I found this site!

 

[macrophobia]

Here's the picture: