Easiest way to buffer  - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: August 11, 2024, 04:25:18 am
content: Hi,I have an interface board with 16 outputs and 18 inputs spread across two 26-pin IDC connectors, I have a problem with my outputs (12 on IDC_1 & 4 on IDC_2), the FPGA I use to drive the outputs only has about 1.5mA on a pin and the optocouplers (HCPL-2631) need 5mA.So would you make up a BOB with two SN74CBT3245C on there and four IDC26 connectors and route the 16 outputs through the FETs and the other pins just pass through?Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2024, 03:16:50 pm
content: Why are you using opto-couplers?Digital isolators don't need such high drive currents.https://www.ti.com/lit/ds/symlink/iso7840.pdfhttps://www.onsemi.com/download/data-sheet/pdf/ncid9401-d.pdf

author: SteveThackery
author_rank: Super Contributor
timestamp: August 11, 2024, 05:51:53 pm
content: I didn’t know there was such a thing. Thanks!

author: rthorntn
author_rank: Frequent Contributor
timestamp: August 11, 2024, 08:27:57 pm
content: Thanks, it's an off-the-shelf CNC board so I'm stuck with the opto-couplers.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2024, 10:22:38 pm
content: To answer the original question: yes, the SN74CBT3245C will work, but is a bit overkill. How about an ordinary line driver IC?Some examples:https://www.mouser.com/datasheet/2/149/74AC245-1006490.pdfhttps://www.ti.com/lit/ds/scas513g/scas513g.pdfhttps://www.mouser.com/datasheet/2/149/74ACT240-888464.pdfhttps://www.ti.com/lit/ds/symlink/74ac16244.pdf

author: rthorntn
author_rank: Frequent Contributor
timestamp: August 12, 2024, 01:01:16 am
content: Thanks, could you please help a beginner out some more and help me understand why (my friends suggestion) was overkill and how your suggestions differ?

author: Buriedcode
author_rank: Super Contributor
timestamp: August 12, 2024, 02:13:26 am
content: If you're requirement is only a few mA, or say 8mA per channel, and high speed isn't of a concern, then a buffer from the logic series HC/HCT/AC/ACT/VHC/VHCT/LCX will do. The 74xx541 has a nice pinout - all IO in a line, as does the bidirectional 74xx245. Where xx is the logic family.the T in HCT/VHCT/ACT is for TTL thresholds, will only work at 5V, but will accept 3.3V logic on their inputs. The LCX/LVC families are 3.6V max, but 5V tolerant inputs, but 3.6V is enough to drive optocouplers.Assuming your IO is one-way - you're using optcouplers so these channels aren't bidrectional, they're either a fixed input, or a fixed output.So, theres a huge range of options there - I would go for something like the 74HC541 or 74HC245, since the pnouts are nicer to work with, and they're cheap. Only 8 channel, but for 16 - just use two!.

author: rthorntn
author_rank: Frequent Contributor
timestamp: August 12, 2024, 02:48:14 am
content: Thanks, I guess the maximum speed on a step pin could be something like 320K pulses per second:(200 pulses per rpm x 32 microsteps) x 3000 rpm = 19.2M and then divide that by 60 to get seconds.Lots of variables, 3000 rpm is more servo and they might not support microsteps, not sure.I see references to a 400kHz stepping frequency on motion controllers.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2024, 08:21:55 am
content: Quote from: rthorntn on August 12, 2024, 01:01:16 amI didn't check the data sheet for the 74CBT3245 properly.https://www.ti.com/lit/ds/symlink/sn74cbt3245c.pdfThe 74CBT3245 is completely unsuitable for your application. It simply an array of switches, all controlled by one signal, on pin 19. When pin 19 is low, then each A pin is connected to the respective B pin, via an internal transistor.Quote from: rthorntn on August 12, 2024, 02:48:14 amLine driver ICs are several orders of magnitude faster.

author: rthorntn
author_rank: Frequent Contributor
timestamp: August 12, 2024, 10:22:35 am
content: Thanks, should I just grab the first one on your list, the 74xx541, any idea if there are versions that gave more buffers like 16?I have a Q over here about being able to switch "around" buffers so I can use this "adapter" on different breakouts:https://www.eevblog.com/forum/beginners/compact-and-intuitive-way-to-reroute-pins/

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2024, 10:36:21 am
content: A 16-bit line driver has 16 channels.https://docs.rs-online.com/5e90/0900766b80024c9a.pdfhttps://www.ti.com/lit/ds/symlink/74ac16244.pdf