What is DH-485?
DH-485 is a protocol that was originally only available in the SLC-500 family of PLC’s via RS-485 ports dedicated for that purpose. However, since DH-485 is merely a protocol it’s not limited to RS-485, and it works well with products that support communicating DH-485 over RS-232. however, when interconnecting multiple devices RS-485 is used because its a multi-drop network designed to support network lengths of many thousand feet, while RS-232 is designed for direct connections and is limited to 50ft (without the addition of boosters or modems.)
A typical DH-485 network is made up of several SLC-500 processors connected to 1747-AIC link couplers. These link couplers act as “isolators” to electrically isolate the communications port on the PLC from the interconnecting daisy chained RS-485 network. The daisy chained connection from 1747-AIC to 1747-AIC is made using Belden 9842, a shielded cable of two twisted pairs and a drain wire suitable for the RS-485’s maximum length of 4000ft, and the DH-485 maximum node limit of 32 devices.
To add our MicroLogix to an existing DH-485 network, we first need to know which models support DH-485, and whether they support it via built-in RS-232 or RS-485 ports:
- MicroLogix 1000 (series C) Channel 0 Mini-Din (RS-232)
- MicroLogix 1100 (all) Channel 0 Mini-Din (RS-232 & RS485)
- MicroLogix 1200 (all) Channel 0 Mini-Din (RS-232)
- MicroLogix 1400 (all) Channel 0 Mini-Din (RS-232 & RS485) and Channel 2 DB9 (RS-232)
- MicroLogix 1500 (all) Channel 0 Mini-Din (RS-232) and Channel 1 DB9 (LRP only) (RS-232)
As we can see above, nearly all the MicroLogix processors support DH-485 via RS-232, while some newer models also support RS-485. In fact, only the MicroLogix 1000 series A and B manufactured prior to June 1996 doesn’t support DH-485 at all.
Since all the models listed above support RS-232, we’ll start by listing the hardware needed to add a MicroLogix RS-232 port to an existing DH-485 network. First, we’ll need a way to convert RS-232 to RS-485, and Allen-Bradley has just the product to do that. The 1761-NET-AIC ($288) is designed to do one thing – convert RS-232 to RS485. In fact, that’s all is does. Many in the past have mistaken it for a DF1 to DH-485 converter, but anyone who’s tried to use it in that capacity knows it doesn’t convert any protocols. However, it does a good job converting RS-232 to RS-485, so good that I’ve used it multiple times when I needed my RS-232 enabled PLC to communicate with RS-485 ASCII devices.
As you can see from the picture to the right, the 1761-NET-AIC has three ports:
- Port 1 (Bottom) is a RS-232 DB9 port
- Port 2 (Top Right) is a RS-232 Mini-Din port
- Port 3 (Top Left) is a RS-485 six position terminal block
Port 3 is the port which connects to the DH-485 daisy chain. If fact, this terminal block is the same TB on the 1747-AIC. So, when retrofitting a MicroLogix and 1761-NET-AIC in the place of a SLC-500 and 1747-AIC, the same terminal block can be used and no network re-wiring needs to be done.
Port 2 is the port we’ll typically use to connect to the Mini-Din ports on the MicroLogix. In fact, if you are using a MicroLogix 1000 (series C or later,) 1200, or 1500, those Micros will also supply power to the NET-AIC eliminating the need for an external 24vdc power supply. The MicroLogix 1100 and 1400 can connect to the NET-AIC the same way, however they won’t supply power through their Mini-Din connectors. In fact, I don’t recommend the NET-AIC for use with the 1100 and 1400 as there’s a cheaper means of getting them on DH-485 which we’ll cover in a minute.
To use Port 2 to connect to your MicroLogix Mini-Din connector chose one of the following cables:
- 1761-CBL-AM00 (17.7 in.) ($49.60)
- 1761-CBL-HM02 (6.5 ft) ($79.10)
At this point the question often arises, “couldn’t we also connect our MicroLogix Mini-Din port to Port 1 on the NET-AIC?” The answer is yes, you could use either the 1761-CBL-AP00 ($49.60) or the 1761-CBL-PM02 ($77.80,) both of which are Mini-Din to DB9 DTE to DCE cables. However, if you did this you would have to provide the NET-AIC with 24VDC, something that’s not required when connecting the Mini-Din on the 1000, 1200, and 1500 to the NET-AIC’s Port 2.
You could also connect the DB9 on the MicroLogix 1400 or 1500 (LRP) to Port 2 on the NET-AIC using the same two Mini-Din to DB9 cables (AP00 and PM02,) just mentioned. And these same cables would also connect a SLC-5/03, 04 and 05’s DB9 to the NET-AIC’s Port 2. However, since the cost of these cables are substantial ($50 and $78 respectively), we could save money and forgo the Mini-Din to DB9 cables and instead connect these PLC’s DB9 port to the NET-AIC’s DB9 Port using generic DB9 to DB9 Null Modem (DTE to DCE) cables from Amazon.
Back to the MicroLogix 1100 and 1400. These two micro’s won’t provide power to the NET-AIC from their Mini-Din port because Rockwell replaced the power pins on the 1100 and 1400 with native DH-485 pins. This means in place of the $288 1761-NET-AIC, we can buy the much less expensive cable, the 1763-NC01 ($52,) and save nearly $300. The NC01, as you can see in the picture to the right, has a Mini-Din connection for plugging into the 1100 and 1400, and the same DH-485 six position terminal block as the NET-AIC and 1747-AIC.
One final note: In this article we discussed the hardware needed to add a MicroLogix PLC to a DH-485 network, but you also need to make one small change in your PLC program. Under “Channel Configuration” you need to set your MicroLogix communications channel from the default of DF1, to the new network setting of DH-485. And don’t forget to save and download you program after making this change!
I hope you’ve found the above information about connecting a MicroLogix to a DH-485 network helpful. As always, if you have any comments or questions please don’t hesitate to leave a reply by using the “Leave a reply” form at the bottom of this page.
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